Mike, it could be too much silverbromide from the previous films you developed.
It is best to dispose the remaining fixer and make a new one, following the instructions from the factory.
Take care to clean out very carefully all containers before putting the new fixer in it, make sure the water is clean and store it in a cool place.
Before you start using the new fixer, take a short piece of undeveloped film, could be a piece of the leader, and put it in a bowl with your newly made fixer. Carefully note the amount of seconds it takes to make that piece of film completly blank, so you could look through it.
Write it down on a piece of paper and tape it on the container of your newly made fixer.
Every time before you are going to develop a batch of films, you do the fixer test again, take note of the time.
After a while the fixer needs more time to make that little piece of film completly blank.
When the time is twice as much than it was on the first time you made the test, it is time to dispose the fixer and make a new one.
Best regards,
LCEL.