If a block is between you and a creeper at eye level, the creeper will not explode, even if you are close enough to make it do so. It always knows you are there, unless you stay there for long enough, so if you back away from the block, the creeper should chase you again. I can't tell if this is intentional or not. I've tested this in older versions, but the farthest I went back is 1.8.9.