Why does my active charcoal Black & Blum stay at the surface of the water ?

 It sometimes happens and doesn't mean your charcoal is not working. The charcoal simply has many different layers in which sometimes air is trapped, stopping it from sinking into your bottle of water. 

One easy way to fix this is to boil your charcoal for 15 minutes, you will see the air escaping from it. Hereby a video to help you.

You already use or think of using Black & Blum active charcoal and you have questions? Here is an article from them - providing broad useful information.