Spawner Light Problem

Hi , I am making a mob defense arena map and I have a problem, I want mobs to spawn each round by a simple mechanic, set a high spawn rate and have a button, the mobs should spawn while the button is pressed due to an inverter. the problem is that mobs will sometimes spawn even when the lamp is on, do I need more lamps or do I need to edit the spawner properties.

Anyone who can successfully fix this will get shout outs in each map update and in the map itself (note it will be the first person to say the working solution)

Thanks,

DMM