Sometimes i can't see well with glasses. why?

Maybe Presbyopia: Usually when glasses work some of the time but not at others it is due to presbyopia or difficulty with accommodation up close - this is usually solved with bifocals. Other times, blurred vision may not have anything to do with glasses like dry eyes or blepharitis. Both of these have intermittent symptoms and can interfere with clear vision.

Recheck the Rx: Have the eye care professional check the glass prescription, they may have been made wrong or too stong/too weak.