It may be that your sizing variables are course enough so that reducing any variable would cause a negative margin. For example, your thicknesses range from 6.39 to 12 mm with 5 permutations. So your possible thicknesses are:
6.39
7.79
9.20
10.60
12.00
It is possible that going from a thickness of 12 to 10.6 could cause a negative margin. Look on the failure tab to determine your minimum margin to see what could be driving this.
For turning on or off failures, simply go to the Failure tab and click on the hatched boxes in the Available Failure Analyses frame to turn failure modes on and click again to turn them off. You can turn on or off all failure analyses under Limit or Ultimate but clicking the Toggle button. You can return to default values by clicking the "Analysis Options..." button and selecting "Get Database Defaults..." from the resulting pop-up menu.
Please let me know if this helps.