Difference amplifier schematic
George is right: your amplifier doesn't work because your input (-0.5 V) is outside the acceptable input range.

In order for the amplifier to work, the NMOS in the pull-down section (M3N, M4P and M7) must be ON and saturated. Therefore the inputs must be at least \$V_{GSon} + V_{DSsat}\$, or about 0.9 V (very indicatively, depends on the devices).

So you either have to use a negative supply or change the input range. Note that using a differential amplifier with negative output may not be the smartest way to shift your signal up by 0.5 V. One thing to look for are level shifters.