Valve is aware of and looking into a bug which causes molotovs to not explode in mid air.

The bug in question makes it so Molotov grenades behave like smoke grenades, not detonating until they hit the ground. This allows players to deny key choke points from anywhere on the map, which alters the entire way the game is played. However, Valve revealed that they are aware of the bug, and are looking into it along with a few others. 

The bug came with the newest update to Counter-Strike: Global Offensive, which added the new battle royale mode Danger Zone. The update also officially made CS:GO free to play, meaning anyone can now hop on and play the game without having to pay the $15 for the game. Players who already owned the game have been reimbursed with prime membership.

Photo courtesy of Valve