912iS Starting

in addition he notes he cranks it for only 10 seconds.  Given this I might think low battery power might be an issue.  Once turned over your priming the pump and lines up to pressure and perhaps it needs 20 seconds?  Difficult to guess with just text.  

I would perhaps test if the battery is low or too small, a simple booster next time might make a difference.  If it does then perhaps up the cranking amp rating of the battery.  The listing size for the engine is a minimum and not what I would recommend.  The new Lithium Ion batteries, with automatic cell balancing, is what I like as they have lots of power and are very light.  (EarthX has a good one)  Depends on the aircraft.  Another thing to do when having an issue with starting, it applies to all the Rotax family, simply try some fresh fuel.  Auto fuels have very different vapor pressures and that might be an issue dependent on the temperature where you are and the available fuel supply.  Avgas 100LL (ASTM D910 fuel) works fine and it never changes vapor pressure seasonally so that is a good yardstick if your having issues.  

Just a few ideas to check.

Thanks for all the informative replies guys, much appreciated. Next start-up won't be until next week so let's see if it behaves any differently. A new battery, like the EarthX might also be the way forward, though here in the UK that does involve an amount of paperwork to do. 

Rotax Wizard, you mentioned "It is important to start on only one fuel pump and one lane."  Based on the Operators Manual dated June 1, 2019 it states to have both lane select switches on during starting (section 3.5, step 5). Have the start instructions been modified recently?

All the 912iS engines I have seen use both Lanes to start. All I have seen also use rotary ignition switches.

I think he was just referring to the case when the battery might be weak... why have two pumps running before startup when you're trying to conserve amps for the starter, etc.