As you mention this horse is green: is this horse fit enough to be asked to accept contact, engagement, bend, etc?
Fitness and strength are two things that are high on my checklist when things aren’t going right. While we try to stay mindful of walk breaks, changing up exercises and movements to work on, my horse still gets tired during some lessons (hey I only get her once a month…so we try to do what we can). His main tell is dropping behind the bit. We try to wrap it up on a good note with some leg yields and stretching when we see it. (It doesn’t happen every lesson…but some days we push a little more)
Just one more variable to the equation.