It's annoying and counter-intuitive.

Basically here's the skinny :

1) When you first get a skill/spell, it doesn't improve as much as when you've had it for several ranks. I don't know if hours also have something to do with it but you will spam skills up faster when you've learned them 3-4 ranks earlier than the rank you're at now - compared with one at your current rank.

2) You need to switch areas after a while. You'll see improvements if you leave the area and go somewhere else.

3) Stay out of your guild. Stay out of your cabal.

4) The higher over your rank the NPC is the more you learn.

There are ways to game the system but its annoying. Your best bet, as usual, is to bribe a higher level char to help you do it on high rank mobs (like while they're ranking providing you flee out when its hurt), than to do it yourself. Alternatively wait til you get all your tanking skills so at least you can fight stronger mobs.