not holding login across sites – temp fix

Hi, I have a main site From that I set up and it was working perfectly hanging off

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 If I logged in, I would be returned to and STILL not be logged into (I could get in from 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.