wither not spawning

got wither skulls is creative, soul sand in survival( i think)
placed soul sand and 2 skulls in survival, then logged out for a couple of hours, when i came back and loaded my save, i placed the 3rd skull, but the wither did not spawn. i went into creative to rebuild it as i did not have a pick to harvest the skulls and it still did not work. this might just be my computer, but i doubt it update: all better, never mind