Hi there, my pigs are also weird like that, but they just go to their bottles when they feel like. Maybe they're just not thirsty. Ironically the two who drank regularly are those who got the bladder stones.
With Geezer, who wasn't a big drinker until more recently, I had found that he loved water jellies. I found some on Amazon Japan, but I think they are sweetened (not sure, as I can't read Japanese and the photo translation thingie gives me different results every time) so not a great option. You can however talk to your vet and see if they can order them for you from a lab supply company. It's just plain gelified water that they use when transporting animals so their water won't spill or so they won't drown in it. Such suppliers only can sell to practices or laboratories, that's why. Maybe it's a bit overkill but if your pigs just won't drink, it's an option.