I've never not been able to get into an arena, even if it's at its max. Sometimes I see a cap of 300, but there's 380 in the arena.
I log into another arena to load the textures and skins. I quit back to the arena menu and most times the cap is reset to 400.
Even if it's full at 400, I'll log in and out a few times and within a few minutes I'll find a slot.
I think many people simply quit trying after one or two attempts. Be a bit persistent and you'll get a slot in less than 5 minutes.
My regards,
Widewing