I've never used mine for self belay or repelling and haven't experienced this. I'm sure you're aware that smaller diameter ropes will slide through the breaking system, however, let me ask you this. What size rope are you using? There was a recall on the grigri 2, although I'm not sure this reflects the issue you're describing.
http://www.petzl.com/en/outdoor/recall- ... t-grigri-2