Audi A4 won't start

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make sure battery terminals/wires are clean and tight , check any applicable fuses for condition and clean any tarnish from the tangs

You say the truck has full power, but does it? It might look that way by the lights... A battery that's low might have enough juice to ight the lights, but not enough to run the starter motor. And if a jump doesn't work either, the bad battery is sucking juice from the donor battery. Have the battery checked or leave the donor battery connected for twenty minutes before trying to start.