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A: You can always follow up with a station down the line. If you have a contact person who's talked to you, give them a call in 2-3 months to see how their production schedule is.
Sooner than that and you'll get blown off or labeled as a pest. The best thing to do when turned down, especially when the producer or your contact person says they're booked right now, is to politely ask when their production schedule might let up.
Or, even better, close the conversation by telling them you'll follow up with them in the future. This leaves the line of communication open and doesn't close the door to an interview down the line.
If you still can't get an interview, send out another press release when you have more news. Did your book just get published in another language? Go to a second printing? Win an award? Contact all of the previous outlets you've interviewed with before and try again to penetrate the ones you couldn't.
Thanks for writing and good luck promoting your book!