I would say this is definitely a cause for concern: a healthy guppy should be out there exploring its environment. A few possibilities spring to mind:
it may be ill
it may be a female about to give birth
it may be stressed out by bullying
it may be stressed out by ammonia, nitrites or high nitrates in the water
Make sure you get those test results. Also, have a look at the fish's general appearance- has it got worms hanging from its abdomen, does it look bloated etc? Sit down by the tank and see how it gets on with the other fish.