Maybe it is because you are not finding the subjects interesting. Or else the problem is with concentration. One needs to develop concentration by practice. It is by practice, by teaching the mind to sustain, to hold attention, to concentrate on whatever one is doing that one can develop the concentration by exercising it. Once the concentration is developed, then the rest becomes easy. So what is needed is to read whatever you are reading, to do whatever you are doing, with full attention. Then you will apply the same process while studying and it will be easy to focus.
Affectionately,
Alok da
