Two things:

If the ambient air temps are cooler sometimes warming the part with a heat gun until it's warm to touch can allow the plastic to be pliable enough to not break off the plastic tabs used to hold things together.

On a somewhat related note the large variety of plastics can make it difficult to repair broken and cracked stuff easily. The choice of adhesive is critical for success.

I recently stumbled across Ultra-violet cured plastic bonder called Bondic. Probably won't work for everything, but it does things that no other adhesive can do as easily and solidly. Material on both pieces, position in proper place and apply the UV light to the material; in a couple 3-4 seconds the material has bonded to the parts and also hardened.

Kind of costly but a $10 glue can save all kinds of otherwise very difficult, hard to repair, expensive and hard to find cosmetic trim stuff. One nice thing is the bond/filling cures in seconds. It's also sand-able and paint-able after cure.