Why is white ink so difficult to print?
There could be several factors involved in your white ink being difficult to print. Before you try to make adjustments, know that white ink has to be thicker than all other colors. If you try to make the ink thinner and easier to print, the opacity and coverage of that ink diminishes drastically. It may even look faded or vintage.
What's the process to figure out if the ink is too thick to print? First, warm the ink up to production temps, which is around 80°F or warmer. If the ink is still too difficult to print, add a little curable reducer to lower the viscosity. Try 1-3% at first and do not exceed 5% by weight. Test print while the ink is still warm to confirm.
If you tried to add a reducer while the ink is cold, you could add WAY too much reducer to make it easier to print and thereby reducing the inks coverage and opacity dramatically.
You also need to know that there are two types of inks: Short-bodied ink like FN-INK™ and long-bodied ink like Wilflex. Long-bodied inks need to be warm to print well and are generally stiffer inks when cold and in the bucket.
To learn more about handling white inks in cooler temperatures, read this article.