You've been given bad advise by the petstore (typical …. most pet store staff are clueless when it comes to the care and husbandry of bearded dragons).
Only one day …. try again later , if this persists for more than 2 or 3 days there might be something wrong.
A 6 month old dragon should be on 2 meals of live insects per day , make sure insects like crickets are NOT TOO BIG (no longer than the space between the dragon's eyes) , and offered salad as well (if you lucky it will nibble on the salad, but it probably wont unless it's used to it).
How much does he weigh ?
In the meantime, working through this can likely identify the most likely reasons for a dragon being off it's food :
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