If loads are applied to a rigid body in implicit static, make sure RB is attached to (merged to) a deformable body or is in initial contact with a deformable body. You can close any initial gaps using *INITIAL_VEHICLE_KINEMATICS. Alternatively, run implicit transient.
I ran into a case where *LOAD_RIGID_BODY was not taking effect in a static analysis cause there was a small gap between the rigid part and the deformable workpiece. An eigenvalue analysis will reveal any rigid body modes (should not have a rigid body mode in direction of load).