"and someone mentioned that Colt's don't like other mags"
Ughh, WHAT!?!?!. I would guess the Thermold ( and no, I'm NOT saying all Thermold are junk) is junk and your rifle will cycle beautifully with a quality mag. There is probably a reason cdnn can't hardly give the Thermold 20's away for $3.99. Try the Colt and get back to us. Also try PMag, Lancer, D&H, etc and they should all work nicely.
Anyways, Colt/NHMTG/Okay are all the same mags with different floorplates, and IMHO the finest USGI mags made. From my experience, the ones with Okay floorplates go to the military, NHMTG stamped floorplates go to general civilian sales at an average quality mag price point ($13-$15), and the Colt floorplates are marketed at a higher price point (usually $20-$30) simply due to brand recognition. People see the Colt floorplate and will pay a premium, even though the NHMTG is the same mag.