tnt doesn't explode

Put the summary of the bug you're having here
tnt isn't exploding when you place a redstone torch under a tnt,
but if you place a redstone torch first and then the tnt it will explode..

What I expected to happen was...:
I expected that the tnt would explode, but it didn't

What actually happened was...:
I was making a trap with a trapped chest and tnt, I placed the tnt down and then I placed a redstone torch underneath it.
but it didn't explode..

Steps to Reproduce:
1. first place a tnt
2. then place a redstone torch underneath it
3. and watch, no Boom!