I had a problem today.. i tried to link a BMPCC on Atem channel 2 and no signal! Tried on channel 3 its ok! After that a try all channel... no signal on channel 2 and channel 7 ... all others is ok!
Which ATEM do you have? Depending on the model, you might want to check page 18 and 53 of the manual. It could be as simple as switching the input. Third tab over in the Settings > Label. Via the drop down, change Input from SDI to HDMI.