So my M-class its unable to be used with two different authentic Mark Bugs Gem rta's. This is rather unfortunate considering the price I paid for these. The issue is when I screw them down into the 510 they get about half way screwed down and pop right out. It's as though the 510 on the GEM is to small or the 510 on the Gem is too large. I did some searching and have found others indicating a small percentage of the M-class have too large of a 510 and do not work with many 510's. I have contacted the Vendor that sold the device and they are not responding. I sent an email to Yihi today however I don't have high expectations as most others said Yihi indicates they need to contact the vendors that sold them it. Also, I have tested these same GEM's on over 6 other mods and they have no issues making a tight fit with the 510. Hopefully Yihi has some type of solution.
If you bought from an authorized vendor Yihi will take care of you. If the vendor does not assist you Yihi will engage another good vendor and get your problem resolved for you.
If you bought in the grey market you probably won't have much luck but you could contact Perry at Varitube.com and he may be able to resolve your problem.