This is becoming rather annoying.  I have an assembly which is fine, works as required when open, but as soon as I insert it into a 'master' assembly it folds up and fails, even when the sub assembly is set to rigid.  There are no mates in the master assembly to tie it or link it to anything already there.  If I save it all and then open the sub assembly from the master assembly it opens perfectly.  

Any ideas?