Matt is correct. OP is using Ilford Rapid Fixer. It is diluted 1+4 to fix either film or paper. But do not use the same working solution to fix both film and paper. Mix it fresh for each application. At the 1+4 dilution, a fresh working solution will easily fix TMAX films in 4 minutes. 5-6 minutes is fine, but significantly longer times can result in bleaching. Proper agitation during the fixing process is important. Many people neglect this. For paper, a fresh working solution of Ilford Rapid Fixer (1+4 dilution) will fix a sheet of FB paper in 60 seconds. It is very important not to exceed the capacity of a given volume of working solution. Refer to Ilford's technical literature.
In addition to using proper agitation when fixing, it is also important to thoroughly mix the working solution. When mixing working solutions from liquid concentrates, it is easy to overlook the importance of good mixing. However, even in liquid form, standard acidic rapid fixers do not mix with water as easily as one might assume. Ilford has a very useful technical publication (on the website) which discusses this. Don't just pour the fixer concentrate into water, give a quick stir and expect it to be properly mixed. Mix it well.
This is a question I have always meant to ask - define proper agitation for fixing? I think the Ilford PDF's suggest to agitate in the same manner as developer (I.E., agitate for 10 seconds per minute), which is what I do.