I've an M21 since 1987. It's had countless thousands of rounds throught it in that time. A few points:
1. Mine, at least, jammed constantly on Stingers until I replaced the magazine. Problem solved. I then compared the magazines and found that the newer one had a much stiffer spring. I replaced the spring in the old mag, and it functions with hypervelocity ammo, too. Keep your magazine springs fresh.
2. M21's seem to be sensitive to how the ammo is placed in the mags. I've seen this with several, not just mine. If you load the mag carefully, you should have no feeding problems.
3. Misfires are a result of the ammo, not the gun every time, in my experience. By the nature of how it is manufactured, .22 ammo is not as reliable as centerire. That's why JMB invented the .25 acp.
4. .22 autos in general benefit from being kept clean and properly lubed, the M21 is no exception.
5. From personal experience, DO NOT disassemble past the basic field strip unless you have a very, very good reason for doing it. There are some small parts under spring tension that are easy to lose and can be a real SOB to get back in place.