Don’t take the short cut
You can only get that forward-down direction when you’ve paid attention to riding your horse with the contact rein, which is the first phase of the training. So that your horse trusts your hand and has learnt to accept and follow your hand.
However, it can be very tempting to take, what seems like, a short cut and force a certain head-neck position on your horse. But when you force your horse into a certain frame the connection will most likely have the wrong direction.
Change the direction in which your horse is moving
Once you feel your horse has an urge to take your hand into a forward-down direction, you can start to stimulate the hind legs a bit more with the movements I mentioned, followed by resisting the forward energy a little bit with your hand.
This way you can start to change the direction in which your horse is moving. You go from a forward-down direction towards a more horizontal direction and eventually, when collecting your horse, to a more upward direction without losing the correct use of the top line.
But every time you want to make sure you have the right direction of the connection. Check what happens when you open your fingers. Does your horse want to take your hands forward-downwards? Excellent.
If not, stop riding that shoulder in or stop riding those transitions and first focus on getting the right connection again. And that’s not by forcing a certain frame, but by riding the body of your horse towards his head, from back to front.