How to select multiple feature in Modeltree (CtrlKey) is not selecting the multiple feature

I have several part files in session. In all but ONE file, I can select multiple features in the model tree and group them. What prevents the ability to select multiple features in the model tree?


Post note:
Re-starting Creo, then the issue for that singular model is resolved.
This phenomena re-occurs every now and then with random models.