I have an older Chinese SKS that's probably 20 years old, and the 30-round magazines I was using (both metal and plastic) have one by one started experiencing feeding problems (shoot two, next round doesn't feed, etc.). When I gave up and re-installed the factory 10-rounder, everything fed fine.
All of the magazines looked okay (lips, springs, mount, etc.), and it doesn't appear that anything is wrong with the magazine catch on the rifle itself.
Does anyone have a good reliable brand of aftermarket magazine to recommend?
The truth is that SKSs work great with their original fixed 10 round magazines, and poorly with anything else.
The only exception is that, if you really, really want a 30 round SKS, you can try to find one of the Norinco SKS-M or SKS-D rifles, which were designed from the get-go to accept AK magazines. But those are uncommon these days, and tend to command a steep premium. ($800-$1000) They're collectibles now.
If an SKS is your only rifle and you're worried about capacity, get some stripper clips, and practice reloading with those. There are a few manufacturers who offer buttstock pouches for the SKS that hold loaded stripper clips, like https://www.strikehardgear.com/sks-buttstock-pouch/
You might be surprised how quickly you can crank off 30 rounds with a little practice.
Those Tapco extended magazines were junk. Throw it away and just except the SKS for what it was designed to be. The Chinese continued to use the SKS well after the AK came along because they determined you could reload the fixed magazine faster with the stripper clips than it took to handload an AK mag.
