Providing their teeth are not a problem, you could try a few things.
You could reduce the amount of pellets you give them, that way they are encouraged to fill up on hay.
How do you offer the hay? My lot are not really interested in hay racks and prefer their hay off the floor. My rabbits have hay as bedding, which helps encourage them to eat more too.
Lastly, you could try different types of hay. My lot love meadow hay but are also quite partial to Stalky 'Dust Free Hay' and oat hay from Oxbow. The Hay Experts do a hay taster pack, you get a load of different hays, so you can see what they like best.