Placed hoppers above and below grinders with chest linking with the hoppers. Placed a grindable ore like iron and into chest and it stopped after reaching the hopper. Grinder is powered but doesn’t accept the ore.
using latest version of tech reborn to date 4/921 along with up to date reborn core 1.16.5 version. Using fabric mod loader along with few other mods but none that should interfere with the mechanics of tech reborn

You need to configure the input/output sides in the grinder. Right click on the grinder, click on the wrench on the left, and select which face should be input/output

Can't reproduce in dev env. Probably you haven't configured input side, as per previous comment.