I had a similar story with my dips. I couldn't get above 15 now matter how much I dipped and dipped and dipped, with or without weights, slow reps, fast reps, nothing helped. Then I left for a 2 week holiday and couldn't dip nowhere, so I began doing one armed pushups instead. The result: my dips went past 20 when I got home and checked.
Perhaps something similar could be done with the pullups?! Doing some heavy one-armed dumbell rowing for a week or two and no pullups at all?!
Just a thought ...