What type of companion did you buy ?

Ive started encountering this after using the adjustable leveling mod as well. They CAN cast spells, they just cant have a lot of Winds of magic stored up. You need to train them, preferably on horses in a seperate small formation and then guiding them to enemies that dont kill them.

Its a lot easier with some lores than others obviously.

I can't say i have encounter a caster companion with no spellcraft, are you sure this isnt due to other mods?