»Spawner not working?

Hey, I put 2 spawns down and for some reason they are working and not working from time to time, its a pig and cow spawner, is there a trick to them? do they work during the day and not during the night?
Please help, I was not sure if this is a glitch or if I am misunderstanding spawners so that is why I posted here, if it is a glitch please look into it.

 

Make sure the spawner is around grass and has the correct light levels.
Would be extra helpful to state which server this is occurring on

 

Basically what melon said.

You need to put the spawner near the grass, not 3 blocks away from it. Be sure it isn't dark as well.