It's quite long lasting, but check the recommendation for the number of prints per liter. You may be getting close. Staining is more of a hazard than inadequate fixing, I find, with TF-4.

It's not clear from your post, but are you mixing a liter at a time from a the 1 l. bottle of concentrate designed to make a gallon? You should mix the entire bottle of concentrate at once to be sure everything is in the right concentration in the working solution.