It's a feature. They changed the way the pickpocket works. Now, every level of pickpocket allows you rob from an NPC of the pickpocket level times 5. So, in your case you can rob from NPC's of level 10 and below.
The pickpocket only allows you to rob a certain number of things (not sure how that is calculated though), BUT things like 30 medium health potions count as 1 object. 4800 gold also counts as 1 object. A weapon is also 1 object.
That's it. After you robbed too much from one NPC you can't rob from him/her again, unless you raise your pickpocket level again.