After you take it the shutter assembly off the body, inspect if the shutter button linkage to the lens assembly, check if its moving freely and properly. Is it not going down far enough to trip it? Or its a bit bend out so its not touching correctly. Do you have to bend something up, down, in out or adjust with screws.

Secondly, cock the shutter by hand and fire it by hand (or screw driver), does all the linkage moves freely. Apply lube if necessary, VERY LIGHT at a time or it will seep in everywhere e.g. shutter and aperture blade.

If you have anymore questions or would like to discuss, feel free to pm me as well.