Throw the whole retractor in very hot water. (I boil up a pot, then turn burner off before putting retractor in).
After a few min, this will make the cover pliable enough to pull retractor out. Then you can take it apart to diagnose. The orientation of the spring and keeper plates is very important, so take pics and notes. There are plastic parts in there that are VERY brittle, be careful.
A weak or broken spring is my guess
To get retractor in, you have to thread a small rope in the bolt hole in the cover to the bolt hole of the retractor. Then throw the cover only in the pot of water. Then using the rope, you can pull the rectractor into the cover.
PS: Chevelle retractors are a whole different animal then Camaro, even though the basic premise is the same.