Mob Spawner Wont Work

let me take a look see...

 

Soooos, I found your set up. Hostile mob AI is smart enough not to spontaneously walk into water. Most people build the cobble stone platform (mob spawning pads) another block higher than the water. Then they place trapdoors along the edges of the platforms. Mob AI thinks trapdoors are full blocks, so it tricks them into walking over them. If you have 2 block wide gap with trapdoors on either side, the mob will randomly try to walk over from one side of the gap to the other and eventually fall down.

As for increasing the chance for mobs to appear, light up every part of your island that you dont wants the mobs to show up. Use F3 to check for light level. If the block you stand on has a light level of 7 or below, a hostile mob has a chance of spawning there. You want to force the mobs to only appear in the grinder.


Here is the inside of my mob grinder to give you an idea.