If it worked well initially, then it's possible that they might have cleaned it but didn't lubricate it (or lube it enough). As soon as you got some fouling and/or bullet lube on/in the gun, it started to hang-up. Make sure the gun has a coating of lubricant on all the moving parts, and I'd even go so far as to put a drop on the feed ramp (then wipe off any excess).
You might also want to try a few different types/brands of ammo, as some guns can be a bit picky about functioning properly with certain loads or bullet styles. I'd recommend CCI ammo in general, and their less-expensive Blazer .22 LR ammo is pretty decent stuff, full-powered yet reasonably priced. It's one of the few non-plated-bullet .22 loads that I will recommend, and it has actually worked very well in a few guns that wouldn't function reliably with almost anything else.
Finally, the early Ruger MK-I magazines sometimes needed a bit of "tuning" in the feed lip area. The little tabs on either side help guide the bullet up the ramp, but if they are too close together, they hold the bullet too tightly, and can be a source of feedway stoppages. Load the mag, and push a few rounds out with your thumb, pushing the nose upward slightly with your other hand (as they do when they hit the feed ramp); if they seem to stick or hang-up as the bullet passes between the tabs, very carefully bend them outward JUST A TINY BIT, using needle-nose pliers or something similar, and then test-fire the gun/mag combo with live ammo.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)