my guess is that you are using a soft lead nose ammo, and its hanging up on the feed ramp.... with a full magazine it is tight and keeps the nose down as the first round feeds..... subsequent rounds are a lil looser and the nose rises enough to get over the lip.

my MkII does the same, i loves jacketed, hates soft lead

My mkI is the same as well.

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.

Hi, I bought mine in the mid 70's used, and had the same problem, I took it back, and they found a burr some place in the gun, and removed it. Look at the feed ramp, and check the clip over well, clean it, lube it well, I had great luck with WD40. Also my gun loves CCI Stingers!

In addition to doing a polish job on the feed ramp, you might want to consider how you are releasing the slide for the first shot. If you are releasing it by using the slide lock, you may not be giving the slide enough momentum prior to hitting the first cartridge to load. Try pulling the slide all the way to the rear and releasing it to load the first round and see if that works.