Im using oracal 3651 to print on.. Not sure that makes much of a difference. You probably need a new cartridge if it is a used up one..You MUST make sure to shake at least 2 x's a day.
I had the same exact issues..New cartride,shake at least 30 seconds.. If not happy with opacity, run the under base at high quality, let dry, then go over with lw/cmyk That is our routine. We place our decals on black surfaces so opacity is very important.
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Scott