panel antenna howto
It looks like there is a phase reversal problem with the null down the center. I assume that the physical measurements say that there is no null.
Your NEC model does not have the connection of the center of each element to the reflecting plane. First put those in and rerun the simulation.
If you still have the null problem, try simulating a subset of the driven elements and first use just one of the four driven elements. Look for the null. If it is there this is your problem Next do two elements that are end to end. Look for the null. If it is there, reverse the phase of one of the driving voltage generators.