On the SIG Sauer design you may need to drive out the roll pins in the slide & remove the breach block & clean the firing pin, firing pin channel & breach block, plus the firing pin safety & spring, These get very crudy over time in there..
All you need to take the slide apart is a punch & a board with a hole drilled in it, In my stash somewere I have the propper Metric Roll Pin Punches but a standard punch will work just fine..
Here's a video that shows what I'm talking about..
I bet there's a bunch of crud in the firing pin channel, even brass shards, carbon & copper residue..
Hope this helps..
Also taking apart the frame is very easy & it should be super cleaned & lubricated on the pivot points, Sigarms recomends TW-25B Lubricant but any oil or light grease will also work such as Tetra..