not holding login across sites - temp fix

Hi, I have a main site laurelpapworth.com. From that I set up communitycrew.com.au and it was working perfectly hanging off communitycrew.laurelpapworth.com.
Then I couldn't login. My Ajax login on the landing page would spin off into eternity and the admin bar at the top would say "login" even though I was logged in at laurelpapworth.com. If I logged in, I would be returned to laurelpapworth.com and STILL not be logged into communitycrew.com.au. (I could get in from laurelpapworth.com admin bar/my blogs/dashboard but I dont want to force members to do that).

I fixed it by unmapping then remapping in Tools. I know that's a really quick thing to do but will this happen a lot?
Thankyou in advance.