It isn't that the washing is poor.
It is that hardened emulsion gelatin holds on to fixer more tenaciously, so you need to wash longer and/or use a wash aid to get that fixer out afterwards.
I still would like to know though what is happening to cause your films to require hardening fixer though. The films listed in your other thread describing your problems with soft emulsion are all or mostly well pre-hardened, and would normally not require additional hardening, unless they have been subjected to something unusual.
The only reason I use hardener now is with materials that I have toned, which tends to soften emulsion.