Hi Chuck,
When you measure the heat slab, are you measuring from the solder pad? Or from the part itself? If the solder junction is not conducting, the slab will still read 12V, while the part will read 0. I'd also run a continuity test between the gnd pin on the Infineon chip and the ground pin on the barrel jack, to make sure they're connected.
As a general word of advice, reflow soldering works much better with parts that have heat slugs. You can get a low-cost stencil made from OSHStencils, by sending them the .brd file.
I hope this helps!