Given the number of tread side punctures in short order, I have to agree with those who suspect you left the original cause in the tire.

I run my finger around the tire when I pull the tube. If I get cut or scratched it's obvious the cause of the puncture is still in the tire.

But, very often, you will not find the shard sticking out on the inside of the tire. If there's any tread thickness at all, the shard can hide in the tread, and get pushed in as the tire rolls, then retract to it;s hiding place. Roll the tire between your fingers opening up nicks and cuts in the tread and flicking out small shards with your thumbnail. Odds are you'll find more than one, but equally likely only one is sticking ll the way through to cause punctures.