Hello how are you
Your ski has what is called a wear ring around the outside of the impeller, over time or when debris are sucked through the impeller/wear ring it becomes damaged, when this happens the gap between the impeller and the ring becomes greater, this reduces the ability of the pump to move water efficiently.
Its alot like a clutch slipping in a car or motorcycle. At this point you will need to replace the wear ring and inspect the impeller for damage.
The wear ring is part #8 in the link below
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How may i assist you further?
The info i provided you is exaclty what happend with your ski, i see this issue every day.