Have you had the tubes checked for nicks or even scratches? The tiniest nick can damage the seal. When I had one go on me they checked things out and smoothed out some tiny imperfection. Another of my bikes got a stone chip which was the culprit. The mechanic said the best way to feel for something is running your fingernail up and down the tubes.
'08 Vee: Suzy luggage, Crash bars, Rad Guard & front mudguard extension, Zumo 550, Oxford Hot grips, Stebel, PC-8, 12v acc, Scott oiler & lube-tube, Remus Revolution Cans, Bark Busters, Phillips Extreme vision +100 bulbs.
'08 Concours 14: Rad guard, Stebel, Oxford Hot grips, Projekt D Screen, Hugger, GPS mount, helmet locks & front guard extension, Zumo 550, Stebel, Phil's rack & Givi V46, Sheepskin seat cover, Phillips Extreme vision +100 bulbs.