Printrbot Simple Metal prints in air: Z axis not referenced

[lin]

Hi,

as mentioned above, my Printrbot doesn't properly find the print bed height. It prints the first layer in air. It does the autolevelling at the beginning though, but the sensor LED doesn't come on, indicating a premature stop.
Using Cura it works perfectly. The distance between nozzle and sensor plane ist saved in the printer already.
What can I do?

Thanks!
Mike

[CompoundCarl]

Do you have a G28 X0 Y0 Z0 in your starting script before the G29?