TNT does not affect gravel in some instances

TNT was exploding/destroying gravel, then it stopped. I noticed that I placed TNT near gravel, one block away, and the gavel was unaffected. I then dug a one block hole into the gravel patch, ignited the TNT, and nothing happend to gravel during explosion. This occurred about 30 blocks from the first layer of bedrock.it also occurred soon after a block of gravel was forced to fall because of TNT