Two most common reasons would be lack of flow through the cleaner and the hose floater too close to the cleaner.
The hose floater is a ring around the hose that can be repositioned, and this should be around 3 feet up from the cleaner.
Lack of flow through the cleaner could be caused by several reasons. It may be partially plugged inside, or around the joiner between hose and cleaner. Also at the join into your system (could be a wall plug or a skimmer vacuum plate). If there's a right angle in your connection, that could have a large leaf caught. There may be an adjuster which controls how much flow comes from the cleaner vs. bypass. That may need to be adjusted. Too much of the circulation flow bypassing the cleaner through an adjustable opening in a skimmer vacuum plate is also possible. Your system may be pulling less water than it did in the past, possibly due to filter needing to be cleaned.
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