To whoever said the areas around the screws shouldn't be the coldest, you are wrong. Plastic is an insulator, metal is a thermal conductor. The plastic parts touching metal will always be the coldest/hottest points because the metal will pull the heat from the plastic. The center hole would be the most impacted as it is the furthest from any direct heat source. Basically the foot works like a heatsink. Now whether or not this is the problem remains to be seen, but it COULD be.
Other things to check:
If you install the left and right bolts, is the center hole correctly centered between the foot and the base?
Is there any sort of chemical on the screws? Lube, machine oil, etc?
Was the foot cleaned before being installed (for chemicals, as if for example there was still machine oil in one of the threads, that could cause issues)?
Are all the threads in the foot the same depth, could the center hole for example be slightly (less than 1mm) deeper and the screw head itself is cracking the plastic?
Are the holes milled to the same size?
Are the holes clean? Are there any burrs or anything else that could cause uneven force distribution?
Were the screw holes predrilled? (To avoid cracking polycarbonate, you normally drill out a smaller hole and then ream it out bigger so cut is clean):
http://www.ameriluxinternational.com/faq.phpQ. Can panels be nailed into place instead of screwing?
A. It is not recommended to nail sheets rather than screw. Polycarbonate expands in hot weather and contracts in cold. Pre-drill and screw panels in place to allow for panel expansion and contraction with temperature changes.
Q. Is it necessary to pre-drill holes before installing polycarbonate sheets?
A. When it comes to the installation of polycarbonate sheets, pre-drilling is a must! Pre-drilling allows for the sheets to expand and contract due to changes in temperature. Failure to pre-drill can result in the sheet warping and/or cracking around the screw.