can't really see your image so hard to comment, but a few guesses.
a) literacy improvements are related to social class, so a given uplift will have more impact on your richer classes and less on peasants/slaves etc
b) pop growth/class change is going on at the same time and can combine with (a) to increase illiteracy despite your investment
c) you are playing a poor faction and its early game. Literacy is a long haul, in a game as Russia it took me to the 1870s (& some help from events) to start to see a sustained benefit
d) it looks worse, but it would be even worse if you didn't try to improve things.
I'd say its your population mix/growth rates that are driving this