Those are USA (a company that makes crappy magazines) mags, and the misleading marking on the floorplates means it was made to fit into a Colt AR-15, not made by Colt.
The symptom you describe is also indicative of a weak spring.
Best solution is to discard, and replace with any USGI mag:
Adventure Line (later became) Parsons (later became) Center
OKay (also makes mags marked) NHMTG (also makes mags marked) Colt Ind.
DLSanchez
Labelle (later became) General Stamping (later became) D&H
Labelle, and now D&H, also made mags marked DPMS, Defense Procurement Mfg Services, Bushmaster, BFI, Quality Parts, and for other manufacturers.
I have also purchased mags made by Labelle, according to the name and CAGE Code on the bags, but with blank floorplates.
These are all good USGI mags. Get some.
(Do not buy Cooper Industries marked mags)