As has been said so far, check the water supply and check water level in the tank. Remove tank and check the ball cock, it has a small replaceable rubber in the valve.
As for square rings in the sliding bowl mechanism, if the sealing water is leaking there will be water coming out of the middle drain hose after discharge (allow a reasonable time (couple of minutes) for the discharge cycle water to clear and then this drain should be dry). The main wearing part in the bowl is the sliding ring; with poor water the inner sliding surfaces of this component wear away and/or get pitted and you will find the corresponding square ring sometimes comes out of its groove, after the bad surface has worn it down sufficiently. Usually the purifier will fail before its scheduled service and you get to know how long after a while.
Also we have had a slack belt contributing to this problem. Although the belt was tightened, the belt pulley was worn out unevenly and the belt moved up and wore away some of the width, lessening the grip and compounding the problem. There was not enough speed to close the bowl. We had to do a major service and renew the belt pulley. There should be an ammeter on the motor starter, you can use this to diagnose drive issues. When this purifier was playing up the current was lower than normal as the belt slipped.
Hope these observations help, been playing with this model for 20 years!