Bad PEI plate adhesion to heated bed
Perhaps this is due to the aluminum build plate warping/crowning as it heats?  I use a PEI print surface on my Monoprice Maker Select (Wanhao Di3), which has an 8.5"x8.5" build plate, and I've had no issues with the PEI coming unstuck.  However, instead of attaching mine directly to the aluminum heated build plate, I attached it (using 3M 468) to a piece of Borosilicate glass, which is then attached to my aluminum build plate using silicone thermal heatsink pads.  The rational for using a Borosilicate glass print surface is that a heated metal build surface is going to warp or move some, as the heat is coming from one side; the glass will not warp and so you have a completely flat surface on which to print (and the heatsink pads help make up the difference in surface geometry).  I used this 3dprinterwiki article as the basis for my glass bed mod, and applied the PEI/adhesive using another source (which I've forgotten, possibly the RepRap wiki).  While the first link is Wanhao Di3 specific, it covers the idea.  Make sure you tweak your z-endstops if needed on your particular machine, as adding the glass plat raises the bed height by several mm.

Please note that I'm theorizing - I did not ever try applying my PEI directly to my aluminum build plate, so I have no direct comparison.  But the use of glass to eliminate build plate warping/crowning is fairly well documented. See also this 3DPSE Answer.