Salt blocks were designed for cows, who have much rougher tongues than horses.
Some horses don't like licking the block and do much better when provided with loose salt.
My four range from "I can't stand that thing" to "I'm glad you don't like it because I'm going to lick it to the ground"
I add table salt to the feed pan of the horse that just about refuses to lick the block.
Also, what type of salt block? My horses are much more inclined to lick a white block rather than a trace mineral block.
I haven't put out anything but white blocks since one of my seniors was diagnosed with metabolic issues and I was amazed at how much salt started to get eaten