When one castor stops touching the ground all the time is that the rubber on the whee

When one castor stops touching the ground all the time is that the rubber on the wheel wearing out or has something gotten out of alignment? My castors are 8 yrs old and I have not had any problem with bearings but recently I've noticed that one of them is not always touching the ground. Do I just need to order a new wheel or is there a way to lower it so it will touch?

When one castor stops touching the ground all the time is that the rubber on the whee