Here's something that has only happened to me once, in Manchester, but has happened a few times in Kampala:

- The driver simply cancels
- In Kampala, beforehand, the driver is always unreacable (I make a point of calling and describing where I am here, because some drivers struggle with navigation)
- The driver is ALWAYS just around the corner from me, at the point of allocation.

That third point has the neurones in my Suspicion Cortex firing like crazy.

Ok, the answer is simple. In Kampala, Uber drivers also take cash. If they're running short on cash to buy fuel, they will sometimes reject a ride on credit card (which is the only option I have).