I have a JailBird, fits snugly and comfortably, and apart from some initial issues with skin tension under the main ring, have had no problems. I am locked in the JB for various lengths of time, at My Lady's discretion, for (so far) anything up to two weeks, let out for a few minutes to wash each day (I'm uncircumcised so this is preferable). A few weeks ago I was released and was washing, and noticed what looked like a couple of narrow bruises, perhaps 2mm wide and 8mm long, on the underside of my shaft. Over the next few hours the surface skin opened, a bit like a blister, and an open wound followed. This healed over a few days. The same thing happened a couple of days ago, I noticed what looked like a row of very small blood-blisters in much the same area as before, which behaved the same way, opening into a narrow wound. This is starting to heal satisfactorily. In both cases, I only noticed them by sight, there was no pain or discomfort, no other swelling etc, the only thing in common was they arrived after a few days locked up, and they run in the direction of the internal cage bars along the shaft. I always clean the JB in very hot water, thoroughly dry it, and myself before refitting. I don't think I have any infections (yes, I have had an STI test just in case) and am otherwise in good health. I don't use any form of lubricant in the device - having once done so and suffered the agony of one testicle slipping through the gap! The problem appears to be some form of bruising of the thin skin, very near the surface, followed by a breakdown of the surface layer. Any suggestions most welcome.
Maybe you should reconsider using some lube. I rub a little baby oil around the main ring and under the cage every night before bed. If you're able to slip a testicle through with lube, your base ring may be too loose. Also, check the inside of the cage in that area and make sure there are no rough areas that you might be rubbing against.