I voted "sometimes". Thankfully, I've become better about reading now - but for a few years it was a nightmare, and a very puzzling one because I used to be a total bookworm who devoured any reading material I could get my hands on. Then all of a sudden reading became tedious, something I resented and would only do if forced to (like for class. And on that note, my heart especially goes out to those of you trying to get through university like this.)
But I think my difficulty was more emotional than concentration. I remember distinctly feeling silenced as I read - the recurring intrusive thought would be something like: why do THEY get to be heard, but not me? If I was reading a bad/difficult writer, this would lead me to mentally tear apart their work in the most angry, critical, unforgiving way (which is not in character for me). If I was reading a good writer - especially of fiction - I would become intimidated by the illusion they were creating, feeling like they were using their craft to insidiously take over me. The "journeying into other worlds" that I used to love so much about books was the very thing that became threatening. :(
By the way, has anyone else here experienced this sort of emotional difficulty with reading?