I had the head off the MX5 recently, I took it into the shop for a skim, they also replaced the valve stem seals, one had come up off the guide, the car was fine for a few weeks, no smoke etc now its started smoking again, I assume the same things has happened and the seal has come off again, any ideas why it would keep happening?

I has asked for new guides but they said the guides were worn but not too bad so recommended not replacing them.

 

If it's the same guide that's lost the seal, then there must be a problem with the guide itself. The seal should snap on, and be quite difficult to remove.

If it does not seem to stay on well, and you don't want to pull the head apart, then maybe some glue (silicone) will do the trick.