Cog slip only on first skid

Today I installed a new miche cog (15t) on the stupid miche adaptor, tightened the lockring etc. However, on the first skid the cog slipped a little. The cog did not "slip back" when I pedalled forward. Next skid no problems with slipping. When I got home from my ride I rottafixed my cog, but the thing seems to be tight...any ideas what happend?

I must note that I only switched the cog, not the adaptor! Did not touch a chainwhip :) had no 'slip' with my old 18t cog...

Lockring wasn't tight enough

Hm... That is what i thoughed at first. But the cog should now be lose right? However, I just could not get my cog tighter when I rottafixed after the slip...

Nah when youd of pedalled it would have tightened it back up again. Just get the cog tight and then tighten up the lockring as much as you dare

Did you remove the lock ring before rotafixing?

The cog could be jammed against the lock ring and threads but not be tight against the shoulder at the end of the thread.

No, I did not remove the lock ring. Just tried to rotafix the cog and tried to tighten my lockring a bit more (did not move). Strange thing is that no other skids after the slip made any 'slipping feelings', and forward pedaling did not make a 'slip back feeling'.

Think I'll try to remove lockring, cog and put things back on. Hope no threads are damaged...

You need to remove or at least loosen the lock ring before tightening the cog. Quite a few times I've loosened my lock ring and find my cog (which hasn't moved at all) is really loose.

That cog was indeed lose when I removed the lockring! All the threads seem fine though... Thanks for the tip!