Make sure the base pin plunger inside the end of the pin is free. If it is stuck down or the basepin isn't fully seated the transfer bar will catch on the firing pin stopping the hammer from moving.
Exactly what I was going to say. I bought a Super Blackhawk with the 5.5" barrel when they first came out. Had this same problem. A little oil helped, but I went ahead and disassembled it. There was rust, probably from leftover bluing solution that wasn't properly removed, inside the hole the base in slides in. Once I got that cleaned out there were no further issues.
Tom
