Keurig Duo plus brews half has air bubbles in tubing

My Keurig duo plus brews a kcup with no problem, but when brewing a pot, it only brews half of the amount selected. I watched the water going through the tubing to the kcup, and it seems just fine however, the water going to the pot has lots of long bubbles. So a 12 cup pot only fills half way. What can be done to fix this please.