Sometimes you have to be careful not to cover the probe with tape or glue. This can insulate the probe and cause control issues.

Tape the wire just behind the probe and if you want take a small piece of tape and tape just the end of the probe to the mat. This leaves the probe exposed directly to the heat and should work out better for you.

I would leave it between the mat and tank for awhile. It can take a few hours to settle it and start regulating to give the temps you want.