Bob,
M14 Thermolds usually need tweaking before they will work well. IIRC I had to take a Dremel and grind a small amount off the bottom inside edge of the right side feed lip on all of them. After that, they worked fine.
I bought several of the Thermolds back during the first AW ban cuz' that's all I could find at any reasonable price at that time. Right now, however.... given that good steel mags are available for $30... I'd not buy the plastics.
Just my 2 bits,
Swampy
Garands forever