part of your problem could be distractions, not everyone can read while a radio/tv is playing or they are the passenger in a card, that sort of thing
you may need glasses, this could be making it harder for you to read and comprehend without you really knowing it
like conri said, it helps to read a subject that you enjoy, it also helps to read well-written books, some books are written with terribly hard sentences to comprehend
and lastly, you may be better off looking for large format books, bigger letters = easier reading