What are the number of elements in x0? If you have more than 49, then your function might never take even the first step because you limit the number of function evaluations to 50, and fminimax needs more than that number to begin its estimation of the local gradient.
If your function evaluations take a long time because you are running a simulation or solving a differential equation, see some suggestions in the documentation.
I think that you had a good idea to plot something. @optimplotfunccount might give you some idea of how many function counts are needed. Basically, I suggest that you get rid of the 'MaxFunEvals' option and try again.
MATLAB mathematical toolbox documentation