Ok yes I have it working now. I thought the project was active once I had an auth token for it, but by active you mean that the project must be open and running in the Android app for the property update to work.
Sorry for the the misunderstanding.
I must have pressed stop on my project before submitting the http property update request.
Is there any reason why the project and the widget for a pin have to be active for a property update to work? Would it be possible to store the pin property on the sever, in the same way that a pin value is stored on the server?
This would make it easier to label pins from the hardware side.