Change up your pretreat mix.
Instead of doing the standard 50:50 ratio
Try:
60-65% Pretreat
40-35% Distilled water. MUST BE DISTILLED. Even filtered water can mess with the chemistry change your results.
Add 2-3 capfuls of distilled white vinegar into your wagner sprayer bottle. The vinegar's enzymes create a smoother print surface and brightens the white ink.
Also, what type of shirts are you printing on? Different brands give you different results in regards to vibrancy of 100% whites.