Stihl MS291 has me baffled. This is the chain: STIHL 3689 005 0081 Oilomatic 26 RM3 81. I have sharpened countless chainsaws in my life using nothing but a file and I have never had a problem until I purchased this saw. I am doing everything right. I am holding a 3/16 file at a 30 degree angle, and perfectly level, yet after a few sharpenings the chain is so dull that it's like trying to cut wood with a butter knife, The depth gauges are fine. I am on my second chain. I got so frustrated with the first one, that I tossed it in the trash. By looking at the tooth it looks like I am removing the tooth's working corner. What could I be doing wrong?