May well be. Barbs are many times more aggressive than platies (the nice guys). It could be that with a greater foothold in your 28 gallon, the barbs have taken command. One semi-aggressive fish can rule a corner of a tank, (or sometimes the whole tank...) but 3 or more may take complete control.
Fish behavior, though, is complicated. The otos in my 10 gallon (2) were quite friendly for a long while, but finally freaked out and now zoom out of sight when I walk by. I'm guessing that with dwarf puffers (who never seem to bother them) as tankmates, they've become a whole lot more skittish (see the "to nip or not to nip" picture in my gallery or
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). I imagine that the same is the case with your platies. They may not really be harrassed, but may have developed a much greater skittishness/more keen sense of self-defense now that they are confronted with more aggressive fish. But heck, I'm sure some ppl will say that barbs aren't that aggressive... IME
, even a moderately aggressive fish (dwarf gourami, kribensis, some barbs) can have this effect on the real docile folks.
I don't think there's any reason to take your fish (barbs or otherwise) back. I'm guessing that if you wait infront of the tank calmly, your platies will come back out to play.
Hope this helps.