Why is one of my new tires losing psi?

I got 4 new tires about 2 maybe 3 weeks ago on my 2009 Honda accord coupe and when I was leaving work and pulled out of my parking spot I noticed the low tire pressure light came on and when I got home I checked the psi and the tire has around 26-28 psi. It didn't look like any damage was done to it anywhere.
Is this normal? Should I just air it back up and put something like fix-a-flat in it?
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