My guinea pigs only eat about 1/3 of the hay I give them. They eat the "good" hay (leafy parts), then play with the rest or use it to decorate their cages :)
(After they play in hay, remember not to let them eat it. There's probably poop and pee on it.)
The only way to make them eat it would be to deprive them of new hay until they eat all of the stemmy hay, but since guinea pigs need access to hay 24/7 to remain healthy DO NOT do that. It could lead to serious problems. I was just making my point that you really can't do anything about it. That's how guinea pigs are.
I would just give them a space to play in the "bad" hay. I have a litter box full of hay just for playing in duirng floor time and they love it!
I always gave a huge pile in the morning and a huge pile in the evening. The soiled hay gets removed each time before the new goes in.
The PIG in Guinea pig means that they are good at leaving a lot of excess. So hay especially should be eaten but then trampled, pooed and peed on and thrown around. It's extra points if they can get the mess out of the cage and onto your floor. If you're not throwing away any "used" hay, they are not getting enough.
I have noticed that once I started getting our hay from KMS hayloft instead of at the pet store they are eating far more of it than before. There is still plenty that gets trampled, peed on, pooped on and thrown around for "decoration," though. Just not nearly as much as before.