If I see that my guinea pig is not being very active during floor time, I will take him out of his pen and let him walk around. He has a blast! He runs laps around the outside of his pen and does popcorns around it. It is hilarious. Also, if I sit down and move to a different spot on the floor, he will run over and stop right next to my foot. It is fun for both my pig and me!
To my pig, rotation is key. I don't put him in the same spot for floor time every day, as he may get bored. One day he will go in the kitchen, etc... This allows him to explore new surroundings and it gets him moving around.
Also, the cage they have may not be large enough. As mentioned on one page on home, guinea pigs need excercise on their own schedules. Maybe the time you are giving them may be convenient for you, but possibly, they could be more interested in relaxing. The larger the cage, the more space they will have to excercise when they want to.
Maybe try various toys. Get toys that make noise, or ones that they can roll around (like a tennis or ping pong ball), or something that they can climb.
Hope this helps you a little bit!