I go through period when I can only read for short periods at a time, and I think it may be stress related. Generally reading gets my mind of stress, but there are times when the greatest books become blah blah blah.
At these times I still read, but for much shorter periods and then do something else instead. Or sit like a lump and stress out
It always passes for me. One thing that seems to help is reading shorter books with a reasonable amount of action and witty dialogue. When I am a slump short stories leave me cold, but a 100-150 page book is likely to get finished and enjoyed in a day or two.
Helen