This could be:
- the less used parts of the bed may be dirty and thus not adherinng as much
- nozzle sag, thus the middle of the bed is closer to the nozzle causing too much adhesion
In general adhesion to PEI boils down to the squash of the first layer (how close the nozzle is to the bed). An eight or sixteenth counterclock wise turn on the endstop should help. Use a business card as a feeler to check the nozzle height. For even more granular control of the nozzle height, use the z-offset to raise (positive value) or lower (negative value) the nozzle after homed… I find that .1 increments !take a big difference on the PEI.
To cure the sag, folks have replaced the rods and bearings with hardened rods and metal bearings.