I have a model 13-3 nickel plated revolver that has less than 200 rounds through it. I received it from an estate where it sat new in the box for years and years. When I got it, I oiled it up good and took it out to shoot it- worked good with .38 special, but when shooting .357 the trigger would stay stuck back and I would have to manually move it forward. It would also not operate in single action from time to time (the hammer wouldn't lock back). It now does it with .38 loads too. Without ammo in the cylinder, I cannot replicate the problem, however, with spent shells in the cylinder, the problem exists. What should I be looking for?
Thanks in advance!

Take it apart and clean it. More likely it has crud built up form those years of sitting. If you don't know how to do it then take it to a smith for a complete cleaning.

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Had it all apart this morning and cleaned it well...wasn't very dirty. It sat most of the last 30 years or so wrapped in wax paper in its box... Still has the same problem.

Does the trigger fail to reset when cycling the gun empty? If yes, I would look for dirt or debris even metal chips or filings in the action. Disassembly and clean to correct. I've encountered several new, unfired S&W revolvers with metal filings and chips inside the action.

If the action does dot cycle correctly with empties in the chambers or during live firing, I would check for excess endshake and rubbing at the barrel-to-cylinder gap. The inability to cock SA and failure of trigger to reset describes either debris in the action, or something else that is interfering with the action cycling.

The problem only occurs with cartridges in the cylinder...flawless operation when empty. Last night with spent rounds loaded, it would cock back 90% of the time. When I hold the gun up to a light to check the cylinder/barrel clearance, I can see a gap there about the thickness of a piece of paper.

Another check to do: cartridge head-to-recoil shield clearance, the "headspace".

The revolver needs about 0.006" clearance for both b-c gap and cartridge head-to-recoil shield.

I would look for burrs on the recoil shield, damaged recoil shield, or minimum headspace to cause the action binding you describe that makes the trigger fail to reset.

Do you maybe have some high primers that are not clearing the recoil shield and binding?