Start by calling the prison and asking what the problem is. Be polite. No matter what you find out STAY POLITE.
If you do not fix the snag with that call, contact the State DIvision of prisons or Dept of corrections. If they give you a run around or stall you, call the best civil rights lawyer you can find and ask them to become involved.
DOn't forget to ask for help from your state's NACDL Affiliate as well.
Money helps but even if you are poor, contact the closest law school and see if they have a post conviction program or at least a professor you can contact.
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The transfer may be due to his health problems.
See the peron responsible for transfers of funds in inmate accounts and have them provide you with an accounting.
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Prison officials can answer this but i would reiterate to be polite and not hostile in tone when calling to get the most helpful information.