That's because there is no place for them. For example when you place a pickaxe on the rack, then you won't be able to place a weapon to the free slot next to it... You probably have some bigger weapons there and there is no room left for others..
You don't say? Huh, well I'd have a slightly hard time believing it, but I'll give it a shot. They're just various one handed swords like the Blade of Winterhold and such, but I'll see if I can't finagle with it some and get my swords up there.
Thanks! I'd love to say I'll report my findings in a few minutes but I've gotta get going to work, so I guess it'll be later today.