I've been hearing this from middle school English teacher friends (plural) for a while. The last ~15 years have seen a decline every single year in reading comprehension, to the point that ordinary middle-of-the-road books from the 80s or 90s are beyond the ability of gifted students in the same grade to understand without great difficulty.

The chief problem seems to be language complexity and especially holding on to a thought for more than a few words. Even something like The Outsiders will sometimes expect you to keep a few plates spinning in your head until the author takes them back from you a couple sentences or maybe a whole paragraph later. This is a skill especially exercised by reading poetry, as it tends to feature a lot of that holding-onto-context through multiple clauses thing, waiting for the meaning to be resolved.

They can't do that.

They also increasingly find perspectives other than the first person, in fiction, uncomfortable to read.