Chromebook - Zero Touch Enrollment issue

[Sanchez]

I'm having a problem with ZTE on a batch of Chromebooks. This is the first time we've tried to use zero touch enrollment and I'm not having any luck.

The devices have all been pre-provisioned but when I start up a Chromebook it doesn't automatically enrol. If I go through manual enrollment the Chromebook marries up with the pre-provisioned record so all the info seems correct. I hope I'm just missing a step and any help would be appreciated as I'd rather not have to manually enrol all the devices!