Is this part at the end of the roller? Photo would help. It sounds like the part you are talking about is the "Z lock". Most Zipdees don't have one. You need to get it from Zipdee. They are usually installed at the side of the roller not spring loaded. (In my case I have one on each end. Working on the spring loaded side requires unwinding the spring which can hurt if you are not careful) Pretty easy to replace. As I remember what I did was: extend the awning, tie off the awning so it does not retract, use a brace such as a stepladder to support the roller, remove the nut and bolt that holds the roller to the main arm, drill out the rivets that hold the Z Lock on with a 1/8" drill. Reverse the procedure for installing your new Z lock.
FYI/ the lever on one of mine broke off years ago. I used epoxy putty to do a "temporary" repair. Still works.
It can also be replaced with a non locking end cap, also from Zipdee.
Sail on silver girl. Sail on by. Your time has come to shine.