keep my hermit crabs alive

I started with 4 hermit crabs in October last year. Since then, one, I know for sure, has died. One is definitely still alive, the other 2, I haven’t seen since March this year. They buried themselves – I suppose to keep out of the cold. I have read that in Oz, hermit crabs hibernate from about March through to August. Two months ago, I dug up the ground to see where my hermies were. One has liquified in his shell. There was sticky goo in his shell…this was my favourite crab…and I think what I found was his dead body…I never found the other crab. I bought 2 more because I like to see some action in my tank…they all dug and hid too. So I went and bought 4 more. I still don’t see any of them except that one single crab from the first set. I’m hoping when it gets warm again, they’ll all come back up from the substrate and make my crabitarium look alive!