Arizona State Withholding

I am trying to find out why there is no state withholding being taken out for a job in Arizona we are currently working on.  Our company is in Texas but workers are in Arizona.  I have updated our taxes for this year and the Arizona SWH is in the taxes.  Our work state is in AZ.   Can anyone please help give me ideas on why they are not taking it from the employees wages?  We have worked in several states that have withholding and no problems with that.