Plastic pistols don't much turn me on, I only own a few. Two of them are Glocks, and I own them because they are about as reliable as a handgun gets.. I Have always used the standard capacity mags that are designed for the gun, though many people use a 19 or 17 in a 26, or a 17 in a 19 without issue. I wonder if when the gun is being gripped, there is pressure on the magazine that is somehow causing the problem, though I doubt that is it. I am assuming they are genuine Glock mags, and not aftermarket? When the extensions were added, were the springs and followers re-installed properly? One thing I would check is your extractor. Make sure it is not damaged, or loose, as this will cause issues such as you are having. The vast majority of reliability issues with a semi-auto handgun can be traced to reloads, or the magazine. It is very unusual to have three or four factory mags that are all bad. Last question I can think of is how is the reliability with the stock ten round mags? If they work fine, then you have found the answer. By the way, you mentioned Glocks not liking oil. Not necessarily true - They don't need to be dripping wet like some designs to stay reliable, but do need proper lubrication between moving parts. A totally dry gun can bind up. Good luck.
Larry