Maharajji answers questions in the Śrī Guru Darśanam, pp. 97-106

Question: What should I concentrate on during chanting? Should it be the sound, a certain deity or a description?
Answer: When you chant, you are chanting the name. And you have to keep your mind on the name only and not anything else, because chanting is of the Name, not of any form, any līlā or anything like that. If somebody is chanting and thinking of something else, say the form, a quality, or a pastime of Krṣṇa, then he does not understand the fact that the Name and Bhagavān are non-different. Otherwise, if one is chanting the Name, then why should one have to think of some other form, pastime or anything else? If one does that, then the Name becomes secondary and the other thing primary. That itself is improper and displeasing to the Name.

On the path of yoga and karma, they consider the sound just a representation of something, and therefore they chant and think of something else. In the Bhakti-mārga, the Name is not a representation of something, but the Name and the person both are non-different. The name is not signifying anything, it is not a symbol. On other paths, they consider the mantra or sound symbolic. Therefore they think of so many things.

In bhakti, however, you chant the name and you also think of the name only, and the name is Bhagavān Himself. Therefore it contains everything in it. Whether forms, pastimes or qualities – everything is included in the Name. So it has to be chanted and it has to be meditated upon. The name is the form of Bhagavān, śabda brahma, the sound form of Bhagavān. There is no difference between the name and Bhagavān. [..]

Question: When we cannot concentrate during chanting, it means that we are committing offenses. On the other side, we cannot do atonement for offenses. So what to do?

Answer: After taking dīkṣā, one should carefully avoid offenses. That is the main thing. Among the offenses, the major ones are disrespecting or disobeying the guru (guru-avajñā) and not having faith in śāstra (śruti-śāstra-nindanam). These are the basic offenses and all the offenses stem from these two. If one disrespects the guru or considers him an ordinary person, then he will not accept any instruction from him. The tendency is not to take instructions from somebody who is an ordinary person. When one does not accept śāstra, then  one will manipulate it. Bhakti however is based on śāstric knowledge, and if one does not trust it or manipulates it, one will not reach the destination. These two offenses must be carefully avoided.

When one takes considers the Name of Bhagavān as an ordinary sound and does not have faith that the name and Bhagavān are non different, there is no possibility of that person to progress. So avoid offenses and continue chanting even if you cannot concentrate on it. Do your best to concentrate and keep at it. If you are unsuccessful, it is not a problem. That is not an offense.

Question: I heard that the Name is the internal potency and when one is initiated into a proper paramparā, then its power can manifest. Previously I was initiated by someone who was not from a proper paramparā, and I felt that this had some effect one me. Is it that when one gets initiated into the paramparā, one is eligible to get the full effect but not before.

Answer: Nothing can obstruct the power of the Name. Whether before or after initiation, the Name always has the same power. Yet if a person who is interested in material things such as followers, wealth, material position , enjoyment and bodily pleasures takes the name, then the Name does not manifest its power. Rather chanting becomes an offense. Whether such a situation occurs before or after initiation, the Name is unhappy. Bhakti devi must be happy with our chanting.

After initiation, if one desires material things, then the offense is greater because now one commits the offense knowingly. In addition, one is disobeying śāstra and disrespecting the guru because that is not their instruction. Therefore instead of achieving the proper result, one will get an undesirable reaction. The Name manifests its effect if one does it properly, otherwise there is a reaction. Chanting is never fruitless. Whether the result is good or bad, a result will occur. This is so because the Name has power in it. It is like a powerful medicine that cures the disease when taken as recommended. If it is not taken as recommended, there will be some undesirable effect. In this instance, the undesirable effect is that one will become more and more materialistic.

When one takes the name of Sri Viṣṇu, Bhagavān’s mind or attention is attracted to such a person. If the person has material ideas or desires some material benefit, whether it is wealth, position or followers, Bhagavān becomes displeased with the chanting and an undesirable effect or reaction manifests.

It is just like a business in which you offer your labor and serve. Your interest is not in serving but in the result, Your interest is not ultimately satisfactory to the person whom you are serving because he knows that your interest is not  in him. He does not mistake your service as an expression of love but knows this to be merely business. If his interest is to have a loving relationship, he will not be happy with such work being done for money. Similarly when you render service by chanting and yet remain interested in something other than the name, then it means that you are just using the Name for attaining some material goal and this is displeasing to Bhagavān. Instead of pleasing Bhagavān by chanting, you irritate him although the Name is dear to him. Whether it is before or after initiation, this mentality is offensive.