This happens due to unresolved issues and several active concerns. Like you may have yo travel or watch some film or go out with a friend or make a girl friend etc. When you have so many conflicting needs and concerns then you can not concentrate on any focused activity whether its studies or any other activity. The logic of poor concentration is very simple and clear. Problem arises because the mind's abilities are limited and we can concentrate on any one activity at a time. Mind has to think if only those things which are most important ata agiven point of time. If you have many important concerns then mind gets confused and doesnt know when to concentrate and on what activity. So if you are studying, on the surface it may seem that studying is an important activity at that time but your subconscious mind knows it is not. You should assess at such times what your most important concern is and focus on that. Complete that activity first and come to study then. Otherwise your mind will keep wandering.