It would be beneficial to post style of rig, make and model. At least put it in your signature.
Problem could be in pedestal.
You need to start by checking the pedestal or where you are plugged in. Verify that the two parallel blades have 240 volts between them.
If you have a genset with automatic transfer switch the neutral contact could be an issue.
Turn off breakers in the rig and verify power coming into rig after checking pedestal.
Are you using any type of adapter?
Are you plugged into a fifty amp shore power receptacle?