Originally posted by Megahurts:

Don't wall in your surtling spawn. The walls are probably preventing the spawning. Just dig so there's water all around it. They spawn, they die, you go hop in the pool and collect. IIRC they spawn every 5 RL minutes if you are in the area.

Our experience has been that walls (at least stone walls) relatively close to the geyser will not prevent the spawns, but a portal being too close will.