At the beginning of the cycle it starts filling with water , but it fills just a little and unit stays in the fill cycle .

Does it provide an error code? Have you put it into diagnostics? Tech sheet should be on the front/left inside wall.
Drain pump may be clogged. Diagnostics would be a good start. If you cannot find the technician diagnostic sheet It is likely a failure of the machine control, since it is supposed to be sensing the load size and determining the proper fill at the beginning of each cycle. It does not look like it is able to recognize the motor RPM’s, so if the RPS sensor is OK according to the tech sheet, then the control is bad.