Going on the bit and lowering the head are two different things. Going on the bit actually has very little to do with the head, despite its name and if anyone has seen my previous posts, they know it's a term I hate due to its misleading nature. Being "on the bit" is a symptom of the rest of the horse going correctly, and whilst the head carriage can be somewhat forced (there's still tell-tale signs), it completely defeats the purpose.
Lowering of the head is achieved through relaxation. Is there a reason the horse is not relaxed in work? Is it too soon to expect the horse to be comfortable working? Is the horse restricted somewhere so that putting the head down is uncomfortable? Is the horse going forward at a steady pace, or is it trudging along and/or speeding off?