Pro ink prints throughout the white area of the print by default. If you want to only gloss the print part use the NWGCO curves (inside of the NWGCO folder inside of your curve folder: drag the curves to the main curve folder and hit the install command).
When you print with a NWGCO curve add a photoshop curve that brings your whites down by 1 point (so anything that is level 255 goes to level 254). This will ensure full gloss optimization of the printed image and zero gloss optimization of the surrounding paper.
The PiezoFlush does take time to get out of the GCO channel (more than the ink channels). You can use the purge procedures here to clear just this channel more: How To Flush Individual Channel using QTR Calibration Mode