No ISO5 in dry heat oven during cool down phase

Our dry heat oven does not comply with ISO5 or grade A during cool down phase, even though manufacturer claims that it should be able to.
HEPA chamber filters were replaced last april, using a specified heat treatment followed by a cleaning cycle. However, after repeating the cleaning cycle several times, it still does not comply to ISO5 during cool down. Manufacturer claims that their filters should not have an issue with particles generated due to ceramic fittings.
The manufacturer advises to keep repeating the cleaning cycle until the desired result is reached.
Is anyone familiar with this procedure?Do any of you have suggestions on how to proceed?

Interesting question.

What kind of HEPA Filter are you using?
HEPA filter with elastic fibreglass sealant and stainless steel separators?
HEPA filter with ceramic sealant and aluminium separators?

First why you think it is not complying with ISO 5 / Grade A?
Did you check the gasket or encloser of the HEPA filter for leak (stress crack)

The particle count in the cool down phase does not comply with grade A. It does comply with grade A during the warm up and depyrogenization phase. So particles seem to be generated during cool down.
The HEPA filter used are filters with ceramic sealant and aluminum separators. I am aware of the risk of stress cracks in these types of filters. We have discussed this with the manufacturer, but they insist that the filters that they use have countered that problem with the type of frame they use and that that could not be the reason of the higher particle count during cool down.
Failure of the gasket would come up during leak testing would it not? Leak test is conform specifications.