It's the first time I'm trying to pressurize magma and send it up in fountains to burn invaders. The plan is as following:
1. Have a big reservoir (several levels) that is filled with magma by pumps up to a few levels above ground. Marked as Nr. 2
2. have a tunnel from that reservoir to holes in the ground were the magma should come out. Marked as Nr. 2
3. cover those holes with hatch covers, linked to pressure plates and opening when a goblin (or heavier enemy) comes near. For testing purposes, I also linked them to a lever.
4. Burn stuff until it's no longer funny


Now the problem is... the magma won't raise above ground when the hatches are opened. It just sits there and is no real threat to invaders (except the clumsy ones that dive in by themselves). So far I've tried turning the pumps off and on again and closing and opening the hatches several times - nothing changed.
As you can see, the magma is pumped directly into the reservoir (small picture at the bottom) and therefore pressurized. It does rise from z-2 to z-1 without problem (the tunnel for the magma is 2z below ground) but does not go from z-1 to z0 (ground level).
What exactly could be the problem here? Does pressurized magma only rise one level or am I missing something?