If you don't have any phosphoric acid, and the other steps still don't clear it completely, try hot (110 degrees or so) citric acid at 2%. This can get sort of delicate, because too strong and too hot can bleach the print.

Another thing I forgot to ask is if you mix in some oxalic acid with your ferric oxalate when you mix it into solution. It really helps promote clearing, plus speeds up your printing times by nearly a stop (My experience is that 4g oxalic/27g ferric oxalate/ in 100ml of water will make printing times about 60% of what they would be with ferric oxalate used alone)

Another, nastier, option is to get some muriatic acid from the hardware store and mix it up at 1-2% for on of the baths. Be aware that it is corrosive and puts off a lot of fumes. Use adequate ventilation and get rid of it as soon as you are done. It will etch metal pipes in the area of the tray if you leave it sitting around in an open tray. Also, do not use it in conjunction with a sodium sulfite based clearing agent without a water rinse in between. The acid will convert the sodium sulfite to sulfur dioxide gas, and the rotten egg smell will not make you a popular person in your living space.

I try to avoid using muriatic completely because of these problems. So far, the other steps I have outlined have worked just fine for even stubborn papers.