What would cause the trigger on my SP101 to hang up on the return.

New gun? Or did it just start happening to a gun you've had for a while?

I have a 3 inch sp101 in .327 federal. When it was new it had sort of a rough feeling hammer drop and a light hitch in the trigger returning to battery. When I took it apart I found a very slight burr on the edges of both the hammer and the upper portion of the trigger. I removed the various burrs with a super fine whetstone and now she's an absolute peach. If you just aquired the gun check that. If you've had the gun for a while then I'm out of my depth on this one.

It is a new gun that has never been fired other than factory test shot. I have installed a reduce power hammer spring, hammer shims and trigger shims but it was hanging up before I installed those. Most times it will return to battery with just a small hitch but occasionally it will hang up completely but not were you cant easily return it, just enough to be annoying.

A phone call to Ruger may be in order. There might be a burr in the works that you could clean up, but it's new gun and should run proper out of the box.

I just did SP101 trigger jobs for a couple of guys. Both of the guns were lousy with casting & machining flash inside. The trigger return piston bore was so rough I wondered if it had been bored on a screw machine. You may have a similar issue, if you're comfortable with the idea, I'd suggest breaking the gun down and cleaning and oiling the action. You could also send it back to Ruger, or pay a local Gunsmith yourself to do it if you don't want to mess with it.