Originally Posted by oldoneleg
One time this happened, the chain was frozen in the low gears and wouldn't shift onto any of the highest gears. This happened right by a LBS. They popped it up on a workstand and couldn't get it to shift. It only worked if they manually pulled on the cable. They squirted a bunch of lube in the shifter and then told me I would have to leave the bike. Being 15 miles from home, I figured this was a long walk in cleats and opted to ride the damaged bike home. One mile later it unfroze. This hasn't happened since, but the other shifting issues persist.
Thanks for your reply
If they squirted a bunch of lube into your shifter mechanism then that can cause the ratchet to slip.
Shifter ratchets work by friction....they are not meant to be lubricated. The shifter may have been hosed anyway so no big deal...but would explain your shifter slipping in the biggest cog.
George