>> max(Fx(:))
ans =
0.145210103674753
>> min(Fx(:))
ans =
0.0155307058204656
>> max(Fy(:))
ans =
-0.00505640320134679
>> min(Fy(:))
ans =
-0.386812925230569
The gradient is the change in values. It is to be expected that the change in values is often less than the range of values that you happen to be plotting. If you stick with the contour interval 0:.1:1 then only the 0.1 level has any data for Fx, and none of the Fy is in the range 0 to 1 so if you use 0:0.1:1 then as your level list then you would expect a blank plot.