Lawrence,
is it just a few polyps behaving this way or the whole colony? What about the colour of the shrivering polyps, do they look normal to you? If all the important parameters (pH, kH, Ca) of your water has checked out; then you need to check for PO3 and lighting.
Phosphate will prevent calcifications and prevent your coral from laying down its skeleton. Even a minute trace of PO3 will affect the well being of your hard corals and most PO3 test kits available to us are not able to test for such small amount of phosphate. I'd suggest you use something like ContraPhos, RowaPhos or a Polyfilter to help you with getting rid of your PO3. Of course, not over feeding will also help.
I'm assuming that your lighting is sufficient to keep bubbles. If that's the case, you need to see if they are shaded by other corals.
Bubble corals need fairly strong lighting, regular feeding, good water parameters and moderate to low current to thrive. Good luck.