Servo problem

I have a problem

I just recently bought an allbot vr408. When I was done building the allbot, 2 of the knee servo’s react wrongly and the other 2 knee servo’s go down like it should be (and the rest of the sevo’s are perfectly normal).
I already centered all of the servo’s seperately, six centre ok but 2 knee servo’s will not centre, one servo will move up, which is the wrong way, the other one barely reacts.

Can somebody help me with this problem?