Command Blocks: /spawnpoint dosnt work properly

I am making a map called Butter Man Brawl. I want to make it so when the player dies, the player spawns in this small tube which contains a pressure plate that the player falls on which clears their inventory and teleports them to the lobby. But when I die, I don't spawn in the tube, I spawn on top of the barrier that covers the lobby. The chat also outputs: Your home bed is missing or obstructed. When I set the spawnpoint in the tube manually, it works fine. But it doesn't work with the command block. I've tried all of the targets(@a, @p, @r), but they don't spawn in the tube.