I have never found a really good weed
It sounds like you have oxalis of a weedy sort. It is an opportunistic weed
for sure. You may need to reapply your weed
killer several times to get rid of it. it has more roots than it has green growth above ground.
Make sure you read the directions on the weed
killer. That is where most people lose the effectiveness. Like watering too soon afterwards and washing it off the leaves.