Z Axis starting at a lower height after first job

I have a neje Max 4 with e80 module and to give you an example.

I run a job with material set to 8mm and z-offset at 40mm, I run the job and its perfect, it goes down to the correct height and cuts perfect. I then change the material over and make no changes to the job and the height is almost touching the material

what could be causing this?

Cheers