Filters lose suction for a variety of reasons most of which are caused by an air leak on the intake side of the filter. This can happen at any place where air can leak into the system while it is trying to pump. From your description it sounds like over a period of time more air leaked into the system and now the water level in the canister is probably below the rotor impeller area and the rotor impeller has no water to pump.
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Free Customer Support After The Sale
Rena Aquatic Supply 1-800-968-7362
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You should contact the place where you purchased the filter or contact the manufacturer Mars Fishcare API at 800-847-0659 for help with the problem.