It sounds like your HEPA filters pull air from the plenum, is that correct? Are they fan powered HEPAs?
If so, I would recommend adding some sort of pre-filter, or duct work system to supply air to the HEPA filters to avoid the HEPA filters loading up (clogging). This will prolong the life of the HEPAs for a few reasons.
It sounds like your HEPA leak just failed, there could be a few causes for this
1 - The HEPAs do not meet requirements, and must be replaced, and retested
2 - The HEPAs have not been appropriately installed. There might be a leak around the outside of the HEPA where it seals with the can/housing.
Here is another possibility:
1 - The room ceiling is not sealed - you might have a drop ceiling, and Aerosol is leaking in from the plenum, through the ceiling, and into the room - this is also unnaceptable, as air is bypassing the HEPAs and going straight into the room from the plenum - obviating the purpose of the HEPA filters.
In any way, I think you need to find and fix the leak (HEPA, seal around the HEPA, or ceiling)