Some of them just don't tolerate it and the fact you've got two that sit nicely means it's not your cutting technique, it's them.
If you can cut rabbit nails then your clippers must be good too.
I've never tried those 'guillotene' style ones or the abrasive 'buffing' ones but I know they're available (the whirring buffing ones would take for ever to work though.
Our boars tend to grab their legs back and give judgey looks especially GP2. I always worry about breaking his fragile little legs
I find their front leg innermost claw tends to curl under if it gets too long so DD and I do the towel wrapping, she holds (i still get bitten though) and I cut two paws worth at a time if they're being arsey. We just get on and do it , and put up with the black looks after (I'm not going to drive 30 minutes to our guinea-vet, I'm an NHS Podiatrist. Imagine the shame: "You can treat vunerable people but not two stroppy hogs" )
If their nails are long, cutting them little and often is best because the quick tends to extend quite far.Clipping the nails makes it shrink back but it needs done in stages (or they bleed)
Vet will give you some tips , but I found they could lob a tablet down my cat's throat with no trouble whereas I lost layers of skin (maybe they hypnotised her)
So was I but the less firm I was the more they struggled and more likely I was to cause them harm. They're honestly tougher than you think and usually only struggle if they feel frightened or threatened. If they're held carefully but firmly they feel less panicked. My only worry now is cutting too far up the claws! Sorted one worry for another!
IME it's always the ones with dark claws that struggle. Smoothpig sits politely and waits for me finish his claws (he's white). Scruffypig and Current Bun (both black) struggle like mad.
With Scruffy, I persevere and get bitten. Current Bun is just so big and strong that I find it hard to hold him for any length of time when he's fighting me, so I do one or two feet at a time over a couple of days. I tend to cut little and often with all of them - it's just easier that way.
Previous ginger bun (bad-tempered female who hated everyone except me and DD's) was a dream. She sat on my lap patiently and would sometimes even hold out her paws for me. Wish she was still around to keep Current Bun in line.
And again Fernie this information about black clawed critters being blighters to nail-cut wasn't shared before I let DD pick two chaps with 27/28 black claws.
Come to think of it, I had a Himi sow (chocolate brown feet) her piglet (mixed pink/brown) and a jet black Peruvian and they were evil to nail-cut. But none as bad as Evil Edna who was ginger with weird purple (grey/pink ) feet. But she was just odd