It could just be their personalities. I have one boy who acts like your piggies, only moves to get to the food, doesn't like floor time, loves laying down on any soft thingy with his legs out and he's been like that from day one. The other boy is very active, zooms everywhere. So long as they are healthy, eating, pooping and so on I wouldn't worry.
To get the lazy boy to at least move a bit at floor time I spread vegies out all over the play area so he at least has to walk to the food. And he does move so that his cage mate doesn't eat it all!
I have a 2 story hutch with a ramp which the lazy boy would not use. After putting in a ramp tunnel, he now uses it so that gives him some exercise as he likes to lounge on top but has to come down for hay, food and water.
All I can suggest is to use their love of food to get them moving a bit.