Glaze getting absorbed by underglaze

Have no idea what's happening here and hoping somebody else does. I use Amaco velvet underglaze which I bisque fire and then brush on the Amaco HF-9 Zinc Free Clear on top and fire it to (what I believe to be considered) cone 7 (1250 degrees celsius). On some colors the glaze turns out somewhat smooth but on most colors it just turns out gritty. Why is this happening? 😭 I apply three layers of underglaze and three layers of clear glaze. I've heard of applying a thinner layer of clear glaze but this seems to not make any sort of difference in my case, turns out the same with even less shine. Also my underglazes turn out so muted underneath my glaze. If somebody knows why this could be happening, please let me know.