The prefix a- in its meaning "without" (eg, aphasia, ataxia) is fairly restrictive.
That's why in- (with its various Latinate assimilations) is used for "not".
I quite like "preliterate" in a facetious sense for adults who have ostensibly learned to read but suck at it.
Literacy does not constitute ability to comprehend; just look at those who botch theirs SATs.
As I recall, back in olden days, English classes had regular sessions devoted to "reading for comprehension". I always thought that that was "funny strange", for what would be the point otherwise? But now I come across myriads (yes, myriads) of people who are literate and incomprehensive (and, for that matter, cashiers who are innumerate — but that's a whole other story).
