cheap
In a previous post, the OP states that he "got it cheap because it wouldn't feed". So this is more than a dust/environment problem, the pistol had issues when he bought it......and he knew it..... apparently. Dust and environment likely aggravate the problem, but the pistol has not run right from the onset.
Another question.......how long ago since the pistol was detail (armorer) stripped, cleaned and lubed.......like down to all the itty-bitty pieces. When was the last time the extractor assembly was out of the gun and cleaned.
Another thought......I wonder if the gun has the correct extractor, pin and spring. I don't remember what extractor parts were interchangeable, but if they are model specific, a previous owner or a sloppy armorer may have installed incorrect extractor parts. Or....it just could be a wonky extractor.
If the gun has been back to SIG, they likely detailed and test fired........
I dunno.