Outposts only have one medium pad. Also you get denied temporarily if you obstructed pads or something... Also outposts and all other stains won't allow you to dock when hostile.
If you're in Solo and the hangars are all occupied, you only have to wait a few seconds and the NPC in "your spot" will take off again, freeing the pad for you. NPCs aren't pad-hoggers like CMDRs can be.
Not usually a problem at starports but I have encountered it quite a few times at outposts. Generally if you hang around in space and keep pinging, the AI will kick someone out (this is in SP) but it can be annoying if you have a 2 minute bonus ticking down, or the wife is nagging the cat litter needs cleaning and you just want to land and come off the PC.
Where you attempting to land on an outpost? If so, there may be an NPC "inside", below the landing pad. I've seen them come up on the elevator from within and then take-off, clearing the pad.