[PsYcHe]

The only thing I found with my factory job was that the 'v' shaped bits on the side of the removable bit had developed a little bit of rust, so it wouldn't seat itself home properly. A bit of work with a flate blade screwdriver (a convenient one in the toolkit, I was away camping at the time) and then it went in OK, but you definitely need to make sure it clicks into place, otherwise it's bye-bye trailer time.