Three infested mines say "blocked" when I try to enter them

I successfully entered the first infested mine I encountered, cleared it, but then when I tried to enter the next three I found, I was instead taken back outside and told "blocked."

Has anyone encountered this? Is this just bad luck or did something happen with my world?

Edit: I think it turns out it was just extreme bad luck. I kept searching for more mines, and the next one I found let me enter.