This happened to me too. With great frustration I wasn't able to SSH in, I even completely disabled the firewall for a little while but that didn't work. In the end I realised that I needed to wait more than 30 seconds for the Turtle to boot. For whatever reason, mine was being a little slow. So maybe try plugging in the turtle, and then wait 10 minutes. Once I did that, the Turtle has been working fine (and booting up after 30 -40 seconds as usual).