I had a similar problems with one of my ball-locks recently. After replacing the o-ring, heating it before sealing and using keg lube I was still unable to get a solid seal. I discovered that the plastic feet on the lid latch were actually much thinner than on all of my other kegs, which meant less pressure was being exerted on the o-ring. After replacing the feet I haven't had any problems. Check it out.
Primary: Edwort's Apfelwein
Primary: Brewingcentres Pale Ale
Primary: Brewcraft - Cheap 'n' Shiddy Bavarian Wheat
Kegged: Brewcraft - Cheap 'n' Shiddy Bavarian Wheat
Kegged: Brewcraft - Enhanced Dutch Lager
On Tap: Brewcraft - Irish Stout