Friends pay is fucked how does he fix it?

So my friend (E-2) is married living in on base housing and was making about $900 a paycheck. His last two paychecks have been $300 or less. Right now we are trying to find out why but I'm looking here for any advice on this matter and how best to go about solving it. We don't have much information on why the sudden change in pay and haven't made much progress on that end of it so we are pretty clueless.