Some rabbits need more encouragement to exercise than others. There can be a number of reasons for this. If you have rescued your bunny it’s possible that their previous lifestyle didn’t allow for much exercise so it may take time to coax them into a new routine.
There may be a few factors affecting your rabbits desire to run around. Rabbits are most active early in the morning and late at night, and are more lethargic in the afternoon. Changing what time they get access to their run may encourage them to be more active.
Are there hidey holes in the run area? If bunny feels too exposed they may sit crouched in a corner rather than relaxing enough to play, whereas providing a cover or box to hide in can make them feel a lot more confident to run around and play.
They may simply be a little bored, providing toys and forage to root around for will encourage movement .