That's what happens to me when I read text books, especially math, physics and chemistry. By the end of the sentence I have no idea how it started. Even other subjects can have that effect if it's dry.
I don't have that problem in fiction, except for the mind-numbingly boring ones, like "Of mice and men" and Hamsun.
It's not a general problem, it only happens with subjects that are hard to get, boring or dryly written.