what, in the vfp default form behaviour causes an application to lose something as vital as the main procedure file identified in the 'set procedure' call?

If I call one form from another (as opposed to calling the same form from the main menu), then the routine procedures and functions called by many forms (and which are stored in the main procedure file) disappear.

I can bodge around the problem by reissuing the set procedure command as and when needed, but I'd like to understand why I need to - especially if it means I'm likely to be doing something wrong

Harry