Have you ever had a pin on a 0.1″ header that you just couldn’t get to reflow properly, especially with lead free solder (which requires a higher temperature to work with)? If so, it was almost certainly a GND pin connected to a large GND plane. The problem is that the GND plane dissipates a lot of the heat from the soldering iron. You can try using a much larger tip (larger tips do a better job of conducting heat that small ones), and/or you may need to jack the heat up quite a bit, but sometimes it just won’t reflow well to form a solid joint. The solution is easy, but you need to keep the problem in mind when designing the PCB.
What you need to add on any GND pin connected to a GND plane (a large area of copper connected to GND) is to restrict the connection to GND to a single bridge, limiting the other areas with layers 41 (tRestrict) or 42 (bRestrict). Just select the rectangle tool and draw a small rectangle beside the pad over 3 of the four bridges, and you should have a MUCH easier time soldering those pins on after the fact. This can also be a good idea with certain large surface-mount parts.