I have 2 Colt D frames that have the opposite problem of your SP101.
They are perfectly reliable double action but can bind when you try to thumb cock them for single action. One is a LNIB 3rd gen nickel detective special and the other is an e nickel Diamondback (that I just sent to Colt).
I'm not calling Colts "junk" though. They are just old and need a little TLC.