I had the same problem with our two horses....they wouldn't touch the flavored ones.
There is one thing you can try that MIGHT help...try the Himalayan salt licks. Those are the ONLY ones Epona will lick, but it has to be the smooth type. The ones that go in the jolly stall horse toys. The ones that look like a rock she doesn't like...guess because it is too rough on her tongue.
Beau won't lick ANY salt lick, even the Himalayan ones....at least, so I thought. One evening I noticed him licking the salt/mineral block we keep in the pasture. So even if you THINK your horse isn't licking it, they may be and you just don't notice.
If you are worried about her fluid intake, just check her poop. It should hold it's form but have a slight/moderate sheen of water over the poop balls. Dry and crumbly poop balls means she needs more fluids, generally speaking.
I would NOT reccomend feeding her salt supplements or such unless you ask your vet first. Free choice salt licks are always best.
Regardless of if they use it, keep one in the pasture and/or stall, nonetheless....that way it is there if they need it...especially since they may be sneaking in a lick or two and you aren't noticing.