Fixer could be old, contaminated, oxidized, or simply full of too much silver. The test I now use for fixer with film is to take fresh fixer, place a dot on exposed film, wait until the dot clears. Then check the clearing time of known fresh fixer: immerse the piece of film in fixer, agitate lightly until the film clears, which is easy to see when comparing it to the cleared dot area. Triple the clearing time is your film fixing time. When the clearing time doubles, it is time for fresh fixer.
The above procedure is recommended by Ilford.
I have timed fresh Ilford Rapid fix and TF-4 to clear film in just under 25 seconds. Fresh sodium thiosulfate should clear film in about two minutes, I believe, but I would like this last point verified by someone who is currently using regular hypo for film.
I use the counting capacity method for fixer with paper.