In my case the opposite as well. In center I actually had normal to low phosphorus. On home hemo I actually am struggling to keep it below high. I watch my diet much more carefully with respect to phosphorus on home hemo than I did in center, and it's always borderline high. I have also had to increase my binders to keep it in check.
You will need to discuss increasing your NxStage volume, frequency perhaps and TIME on the machine. Many doing 8 hour treatments with 20 L found their PO4 dropping too low. TIME is the most important factor when looking at middle molecules and PO4 but by far, this is not the only factor.