I just had my turn signal mechanism apart last week. There is a small roller on the back of the mechanism, which goes up or down, depending on the direction you select. This roller goes over a small cam I believe. Have a look if this cam is not worn out.
Also, if the turn signal cancels too early, or even will not hold when selecting, the cancelling cam can be positioned wrongly. It should be with the open side opposite of the cancelling mechanism.
1958 T-Bird "Trovão Rosa" - "Rose Thunder"
Thunderbird registry #61670