Disassemble and clean the magazines too....also, as was said, check to see what followers are in them...if they're 2183 they should still feed ball ammo...those followers mostly had issues with HP ammo and usually on the last rd...but replace with newer followers for absolute reliability...call Glock and they'll send you new ones if that's what it is.
AFTER cleaning the mags, check the mag spring and follower = While holding an EMPTY pistol, insert an EMPTY magazine and pull the slide to the rear in a quick maneuver, if it locks the slide back, the spring and follower should be in good enough shape, if not, replace...however, since you didn't mention the slide not locking back after the last rd, they're probably ok but still may need cleaning.
So, if cleaning the magazines doesn't correct your problem, a Glock armorer or at least someone highly familiar with Glock pistols needs to look at it....on the bright side, it can't be much and on the outside chance that it does have some major problem, Glock will fix it for free as long as someone hasn't messed with it in a way to cause the problem and even then, it still won't cost much...
And, just to satisfy the last possible cause...shoot some brand named brass cased ammunition...If Blazer aluminum was what you were using or even if it wasn't, to eliminate that as a possible problem.
Most malfunctions are usually caused first by "dirt"= filth and/or either lack of, or too much lubricant, second by ammo = out of spec, and last by parts failure = broke or worn out of spec.
Another thought, is that the extractor channel could need cleaning which could be causing a problem with either feeding or extraction if it's not moving freely....do a detailed cleaning of the slide....instructions are posted on the internet....check the Glockmeister website under "free tech info" for instructions or find a Glock armorer or someone familiar with this procedure...it ain't hard at all to learn to detail strip a Glock pistol...all it takes is a punch and knowing how to position the parts.