Bent impeller cover.
Possibly might be down to the filter running too hot. Low flow due to long a period between cleans causing motor to run hot. Tank getting too hot during summer. Maybe the cover just isn't man enough for the job. By the way when you put the cover in the hot water hold it by the clip so as this part of the cover does not get hot. 20 to 30 seconds in the hot water is all you need to soften the plastic.