If the engine is inverted it tends to be harder to start for a few reasons - you need to keep your finger over the carb and turn the prop quite a few times to first fill the carb enough to the point that fuel starts to flow down the crankshaft into the head. Then you need to make sure your glow driver is in good shape, as the glow plug will be sitting in a pool of fuel. Of course, too much of a prime and there'll be too much fuel over the plug. So it's tricky to find the ideal prime, but once you work out the right procedure, you'll start it every time.