DIAGNOSIS The filter frame has warped or the filter retention clips inside the unit are broken.
FIX Remove the filter. Check its cardboard frame for moisture damage or bending. It should be perfectly rectangular. Inspect the two white plastic filter guides inside the unit on the left and right walls. Ensure they are not cracked or detached.
IF THAT DIDN'T WORK If the filter is genuine Winix, it should fit. Aftermarket filters often have dimensional flaws. Use only Winix model # AHP550-2 or TK-550-2 filters. A warped frame is a defect.
HEADS UP A loose filter creates an air bypass, drastically reducing cleaning efficiency. It can also let unfiltered air leak past the seals.