Did you check to see if the stems or branches on the sick side got crushed or damaged? Petunias are so delicate that just a little pressure can damage the stems and they don't recover. Maybe you need to cut it back a little more on that side.
If I could see it I might have a better idea. As Tiger said, at least it's perking up on one side already. That's a start!
(do skréše) Jak chceš bét decke zavčaso opozorněné na nénovjéši zpráve ve tvéch oblébenéch klobech na mloveni, možeš je stáhnót RSS kamošem s pomocó RSS ikone v pravym hornim roho léstko klobo na mloveni. (pauloaguia) (okázat šecke vechetávke)