I don't use them but I just quickly tried this too, expected a spawn and no deal. Not about to worry about why
Maybe just use a trigger zone in the interim?
1) Give zombie a name in properties, spawn set to no
2) Add trigger zone and place zombie name from step 1 in If Used property of zone.
If you make the zone big enough and place zombie to center you will get the same effect and also be able to run the zombie on it's own script without switching after it spawns.
Development/ Gaming Rigs
Sys 1: i7-4770 (3.5)/16Gb/128 SSD/3Tb/970gtx/2 x 23, 1 x 27 LCD - Sys 2: i7/8Gb/670gtx/1.5Tb/1 x 23 LCD - Sys 3: Amd Quad/8Gb/645gtx/1Tb/30" LCD