Just because you can fill an AR15 with mud or run 5000 rounds without too many jams doesn't mean you can't dirty it beyond well operating, or prematurely wear it out. Its like smoking, only a small percentage actually get cancer but that doesn't mean smoking is unrelated.
As not only I indicated likely this rifle has a plugged gas system or something else gunked up.
One of the great things about an AR is they are easy to clean completely. If your rifle is kept up with it takes 20 minutes doing a fairly complete job and is a basic part of gun ownership really.
Sure if I shot mine every day I might clean it weekly but typically I shoot on Sunday - when I get home with an AR the upper is removed, BCG removed and stripped down, extractor and pins placed aside (because of the o ring), rest of BCG parts take a kerosene bath. Later on I spray out the upper, gas tube, and barrel with a solvent, cleanup any deposits, clean the chamber and barrel, swipe barrel with oil, scrub and solvent spray off the BCG parts, inspect/assemble/lube.. lower and buffer tube gets a spot cleaning and quick lubes, and it's put away in perfect condition... same with my pistols.
On a piston gun like a mini, or a shotgun that sees low round counts, I may just do the barrel and a once over to remove deposits without a full tear down but generally decent firearms are quick and easy to completely clean and they all work best that way IMHO.