My reading comprehension is allright, I guess. But my attention span is somewhat too short, which means I often read books back to front or just at random odd places.
The reading comprehension per se gets bad only when the text is too abstract. I can handle some philosophy, like the ancient Greeks, most Existentialists and anything connected to practical ethics (analysing situations and the like); all this is simple, follows the rules of basic common sense logic, and (often) involves my emotions, which makes everything much easier. But when it comes to Thomas Acquinas, Hegel, Kant or modern philosophers like Derrida or Foucault, it's no go. I will read a sentence and maybe understand it, or it will seem so, but then I start reading the next and lose the train of thought. The next sentence makes it even worse. By the time I finish the first paragraph, the text loses any semblance of coherence, and I end up feeling like I am wandering in some dense fog.
Math is pretty much no go in any shape or form. Some time ago I translated this large paper on the introduction of innovations into society. The second part was intended for specialists and had lots of sophisticated calculations. I literally couldn't understand anything and had to rely on the grammatical and morphological links between words to be able to translate it. (it was a good linguistic exercise, though )