I'm pretty sure it's hard-coded so that you can only pickpocket once. This is to reflect that NPC's become more vigilant if they notice their stuff gone. In Patch 0 they started to search nearby if they noticed they'd been pickpocketed.
It's nigh-impossible the check anyway since you can only get Thievery to a maximum of 3 anyway, and that is IF you scrape up enough XP to reach level 8.
Use items. I got Thieving to 4 on my character. Course you gotta get lucky with the item spawns, I managed to luck out with 3 different items that give thieving.
