Do you weigh your guinea pigs each week as part of routine care? If not, then please do start. Weighing them each week is the only way to know they are eating enough hay. You cannot judge hay intake by eye. If they are losing weight then that can indicate an illness. All guinea pigs need to eat hay
Try feeding hay in loose piles in the cage or in a litter tray rather than using hay racks. They can find hay racks hard to use. I certainly find my piggies eat a lot more hay when it is right by their faces and they can run through it rather than using hay racks.
Perhaps also try a different type of hay
what I can say though is my piggies will finish a pile of grass but they don’t usually finish a pile of hay. Mainly because the pile of grass I give them is much much smaller than a pile of hay I would give them