Often, when I constrain something in an assembly, Datum Plane to Datum Plane, Datum Plane to Datum plane and a third datum plane to datum plane pair, Pro/E will conclude that the constraints are invalid when I click the final pair.
I don't choose the Align or Mate for each one, Pro/E does. I don't choose Coincident or Offset for each one, Pro/E does.
Why, then, can't it choose a combination that is Not Invalid?
When setting constraints for a component in an assembly, choose default constraints that work, not Invalid, mutually exclusive constraints. If necessary make the defaults intelligent in order that this happens.