EDIT: Changed the title to make it a bit more descriptive.
After a lot of hours playing, I just realized that the reason why it was so difficult to find new spells for Haer' Dalis, Aerie and Jan is... That they've already reached their maximum number of spells (11) in most levels! This was so frustrating, I've spent hours carefully checking all the spell stores looking for the green highlight that indicates that a spell is available for them to learn.
Had I been aware that they had reached their limits, I might have removed the less useful spell in each level to make sure that the game highlights all the spells they don't have. However, I'm starting to wonder if even that is a good idea. I mean, at this stage (I'm currently beginning ToB) they have already filled up half of their spell levels. This means that, in order to keep the green highligt useful, each time I go to a store I need to fully review each of their spell levels, check if it has already reached the 11 spells limit, and if it has, decide which one of their current spells is the less useful one and remove it just to reactivate the green highlighter. That's so much micro, and not a lot of fun!
I think that a mod would be very helpful here. Something like an improved highlighter, which, apart from the current green and red highlights, uses a third one, say, amber. The amber highlight means that your wizard doesn't know that spell but they have already reached their spell limit for that level, so you'll have to get rid of an old spell if you want to learn this new one. On top of that, it might also give you a warning each time learning a new spell means you have reached the limit. And also let you know in the spell book the levels that have reached the limits already (by greying those out, for instance). Such a mod would save hours of micro. Unfortunately, AFAIK nothing like that has been done. I'd love to do it myself, but I'm just starting to learn the ropes of modding, so such a mod is most likely way above my current modding level.
Until such a mod is created, I'm thinking that the best I can do is just using The Tweaks Anthology to remove the limit of spells per level altogether in order to save myself hours of boring micro.