If you fail a Seargent Instructor inspection, it is natural. In my first inspection, I failed because I had rust in the butstock of my rifle. This was a common theme. They want to send a message that your rifle must be IMACULATE. If your rifle is good to go, then they will ignore other aspects.
Also, I passed an inspection with the SI's simply because I did not laugh while one of the SIs was ripping on another candidate next to me. Meanwhile, a candidate that passed the inspection, was failed for laughing. His inspection was failed (although he recieved a passing score prior). Only 10 people in my platoon passed.
When you do weapon maintenenace, make sure your weapon is PERFECT.