I tried to us the mob shard on a slime and it worked fine, but then when I put it into the farm it did not work, other entities still worked just not a slime. (I did try looking at the block data but it showed it was active.)

Hello,

I just tried with a slime in a netherite mob farm and everything seems to work. Can you give me more details (type of mob shard used, type of mob farm used) ?

So, I did some testing and it seems that it will only work if you capture the smallest slime (which may be intentional idk). This is probably because the conditions of the slime loot table.