Let's not confuse tests for residual silver, which test for adequate fixation, and tests for residual hypo (fixer = thiosulfates), which test for adequate washing. A print can be adequately fixed, but not adequately washed and vice versa.
The OP is testing for residual hypo using, I assume, the HT-2 test.
@OP: There are a number of reasons why the Ilford sequence will fail. It's really dependent on the right conditions. Temperature, fixer type (hardening or not), water hardness, the specific paper used, and, especially, carried over fixer in the second wash water, etc. will all skew the results. That's why what you are doing is so important. Test, adjust and find a regime that works well for you in your situation.
While you're at it, do the ST-1 or selenium-toner residual silver test too.
Best,
Doremus