I'd suggest clean it thoroughly, then reassemble without the rebound slide spring. Then, slowly cock the action with finger pressure replacing the missing rebound spring force and watch carefully to see what is contacting what and stopping the hammer going back.
Do it many times until you are sure you understand the sequence of moving contacts and handoffs from one part to another. When you think you see what's going on, look very, very carefully at these contact points and see if they are worn, new, filed on, evenly wearing, and understand what is or is not happening. Above all, go slow and if tempted to stone or file, go very slowly and retry. Some parts are case hardened, only .006" -.007" thick, and if you break through the case, the part is ruined. If you remove too much or make a wrong angle from a non-case part, it is ruined.
Go slow.
There is a set of needed angles and clearances between the trigger sear, the hammer sear and the double action sear to enable the hammer to cock and remain cocked. We're talking thousandths of an inch here, and if someone has been in there with a file, or replaced and perhaps not filed or stoned the new part in the correct amount, angle and places, the action will block up without complete cocking or else may not lock securely in the sear notch at full cock and allow the hammer to fall or push off.
If you want to mess with it beyond cleaning, I'd suggest you get a copy of
Kuhnhausen: The S&W Revolvers manual 5th Edition for best updated instruction, and also
Kuhnhausen: The S&W Revolvers manual 4th Edition because it has IMHO better illustrations, while 5th edition has better text instructions or vice versa, I forget which is which.
These are the Bible for almost anything in S&W revolvers.
I'd post the appropriate Kuhnhausen pages here, but I can't because they are covered by copyright.
There are also threads here discussing sear issues and a search will lead you to some.
Remember: Go slow.
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