If your follow-up inquiries are ignored, send an appeal letter. According to the FOIA, an excessive delay in complying with a request constitutes what's called a "denial in effect." Keep records of your written and phone correspondence with the agencies. These can be useful if you have to write an appeal or go to court.
Technically, the FOIA entitles you to go straight to court if the agency does not produce the records within the time limit, but unless you have a pressing need for the data, you should send a letter of appeal first.
"Your description of the requested documents is inadequate"
If you get this response, it means you probably did not give enough specific identifying information. Give the agency the benefit of the doubt and rewrite your request. You can try to call or make an appointment with the official processing your request to get more help.
"The requested material does not exist"
If you are reasonably certain the records you've requested do exist, and if your request letter was clear and informative, you should try to do more research. Are there news reports, congressional hearings or court records that describe the information you want more clearly?
Rewrite your request, giving the agency more guidelines and clues for where they might find it. Try to be as patient and understanding as you can; some agencies are short staffed or have disorganized filing systems.
