Deep Mines Question

Ok, so I unlocked Digbot as my first evo since I wanted to check out the deep mines. I'm a little confused and worried about how its ability works however since I unlocked one deep mine on my first try and then walked into 10 different mines I had opened or opened up and didn't unlock a single new one. Does that mean im having phenomenally bad luck and can't re-enter those for a new chance? I wanted to find a different island to fight the miniboss with since My Digbot is weak to the poison spirits the firdt 2 waves have and its frustrating that this seems to work like this.