What is the sq ft and size of your unit? My guess would be a poor fitting duck is leaking air. Can you measure the temp at each outlet so see if one is a problem?
I do agree having the system inspected by an independent professional is a good idea.
I could also see a warranty complaint before you hire someone, especially if your cooling temps vary a lot by vent.
If you can see if the air leaving the heater is cold so you know the system is producing cool air going into the ducts.
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