Hi John
I downloaded the source code for the GrovePi api from github and changed the reference in my project from NuGet package to the source code project and started debugging. Both calls worked from the source code. I then changed the reference back to the NuGet package, to confirm whether it the NuGet package at fault. But both call started working there too. So it seems building and using the GrovePi api locally fixed the problem, but I have no idea why.
May be this will help you in finding the reason why.
James