I have a problem with one subobjective where the resultant force equals 0, ¿what can I do to solve it?
good question! That is a classic local minima problem. The first step to tackle with it is to detect it on your code (that is easy, the robot is not at the navigation target but the global force is zero because repulsive force compensates attraction force. The second step is to activate a different navigation mode, for instance a different adhoc balance between repulsion and attraction forces for that iteration. Anyway, in this exercise if you properly adjust the general weights of repulsion and attraction there should not be almost any local minima situation.