Hmm sounds like the bed's not perfectly level.
Or your first layer speed is too fast, or your starting z-gap is too large, or the magnetic mats just aren't that sticky.

Get some dimafix. I find A quick wipe with that and everything sticks and releases well when cold.
One usage lasts for a few months as well.
I'm still using the free 20 ml samples I got 2 years ago from the tct show :-)