I am also pretty sure you can't pickpocket items off mobs that are under level 20 (presumably) due to a bug.
https://github.com/EQEmu/Server/blob.../npc.cpp#L1855
To steal an item, steal_skill has to be over steal_chance. But steal_chance is just steal_skill multiplied by 100 / (NPC's Level * 5 + 5). If the NPC is over level 19, steal_chance will be multiplied by a number greater than 1, otherwise a lower one, so you can't steal items off NPCs under level 20.
I believe I tested this some to see if it was fixed here and came to the conclusion that it was probably not, but honestly I have no idea what that code is supposed to be doing. There are not a lot of NPCs under level 20 with things you might want to pickpocket anyway.