I'm a student of Goenkaji too, although I do not attend courses as regularly as I used to--not because there's anything wrong with it but because my life-practice has developed in a different direction.
Bhante makes a good point about being more flexible with your practice. The conditions outside of the course are quite different. We have day-to-day responsibilities and it may not always be possible to meditate as much as we do on retreat. I understand that Goenkaji advises us to sit for an hour in the morning and in the evening. This is a good ideal to aim for but it is also helpful to understand that this takes time. It can take years for people to develop this routine--and even if one doesn't develop this routine, it doesn't necessarily make one a 'bad' meditator. As Goenkaji says himself, 'Continuity of practice is the secret to success.' So, just do what you can to maintain a continuity of practice. Even if it is 30 min a day or even if you skip a day or two--just 'Start again' and maintain a continuity. It is unhelpful to be upset with oneself as this is just a form of aversion.
Also, there is nothing wrong with doing more anapana. In fact, there's nothing wrong even if you ONLY do anapana. The observation of the breath is a very important and powerful part of Dhamma practice. People tend to forget that the first three days of the course were devoted to anapana.
You mention that your observation of sensations is weak. What do you mean by this? Do you mean that you are not feeling sensations or that some parts of your body appear to be 'blank' when bring your attention to it? If you remember, Goenkaji says that it is NOT about seeking any particular sensations. It doesn't matter if you feel subtle or gross, pleasant or unpleasant sensations. The aim is to move your attention through the body with EQUANIMOUS OBSERVATION. As long as you maintain equanimity, you are practicing in accordance with the Dhamma. Or by 'weak' do you mean to say that you get distracted easily when you bring attention to certain parts of your body? If this is the case, anapana is helpful because it will calm and sharpen your mind. For me, when I'm busy with day-to-day responsibilities I often do only anapana. Like I've said, there's nothing wrong with it.
Regardless of whether you observe the breath or sensations, I'd suggest you avoid projecting expectations onto the practice. Also, you could try to attend a group sit if you cannot attend a full course. Sometimes, meditating with others, even for just an hour every week or fortnight, could help to rejuvenate your practice.
All the best!