I had an aftermarket setup where the HID/Xenons were installed correctly and worked for years.. then one side started going out intermittently, almost the same way you describe. After chasing wires, ballasts, and moisture from the lens, it finally turned out that the 12V battery was weak. It eventually completly died, and when it was replaced, no more HID issue.
The logical reason behind the experience was that the voltage made it through the relay and harness quicker to the first ballast than it did the other and by the time it got to the second, there wasn't enough left to ignite it.
The logical reason behind the experience was that the voltage made it through the relay and harness quicker to the first ballast than it did the other and by the time it got to the second, there wasn't enough left to ignite it.