Thumbcocker.... best way to see what's going on is to disassemble revolver. So-called "old model" Ruger single actions use a plunger loaded by a small coil spring to rebound the trigger. If the coil is rusty, weak, broken, or clogged, it may lack the strength and length to properly hold the trigger forward.
Another possibility, the SEAR TIP on the TRIGGER and be rounded, worn, or damaged. Or, the FULL COCK DOG on the hammer rounded, damaged, or improperly stoned.
Good time to strip clean the revolver, lube and reassemble. If the trigger spring is shot, replace it.
David Bradshaw