777, I have found, can be fraught with problems, depending on batch run, etc. Having run into this problem myself, I have mostly stopped using it.
One technique I have used to remedy the problem in the distant past, (the best would be to reprint), is to create patches (copy/paste) of the areas where the offending area is, make several of them on an image, then print on the back side of a piece of gloss or semi-gloss paper. The swatches won't dry soon, giving you a lot of work time. There is also the end chewed toothpick which does quite well filling the void.
As discussed, I no longer would fill any void - it can come back to bite you, but if you must-there is one way above. It will provide the most faithful colors.
It is quite possible there is some kind of crud that lifts off after printing. You could attempt to roll the canvas out and brush it first, before printing. In lieu of that, use a fine brush head on a vacuum cleaner and go lightly over the surface.
EDIT: Ken beat me to it while typing