@flomadeWill the white ink dry on other kinds of paper?
When you say the ink wipes off easily after a few days, is it dried ink that just didn't adhere to the paper or is it still liquid after a few days?
Is it extremely humid where you are working?
If the ink works on other papers, then the ink is okay.
If the ink is dried after a while, but can be easily wiped off because it is not adhering, try spraying the finished piece with a fixative, like Blair Matte Spray Fix.
If the ink is still wet after a couple of days, you could try drying it in a warm oven (say 130 degrees) for an hour or so, or use a blow dryer on gentle/low. Try this on a sample piece first, to make sure the photo can handle the heat.
If you still can't get the ink to dry, you might try spraying the fixative on the photo before you write on it.
In any case, I would spray the fixative over the BPW any time you use it, to make it waterproof and smudge proof.