Have you checked the soil for grubs. In the SF Bay Area the weather is kinda similar to yours and grubs there can be a problem. It could also be the sunny spot and being a little stressed out from being transplanted. You may want to move them to a shadier spot.
Derick

New Guineas need perfect drainage, a porous potting medium, and will not tolerate soggy soils. I wonder if perhaps you accidently overwatered your plants? Since they were newly planted, they have quite a bit of time to go before their roots fill out those containers. That means that over watering can cause stagnant soil. Unfortunately, these plants can be very quickly affected by pythium and verticillium, either of which would cause wilt.

I've found that they love plenty of sunlight, though some protection during the hottest part of the day.