We have had odd and annoying intermittent starting problems. Only thing I will add that may be of some use to some is this: Although it is officially and true that the throttle should be set to idle for starting, we found that if it is barely nudged a tad higher than dead bottom idle that it pretty consistently starts easier. Of course whether that could be relevant depends on details such as where your idle is set... exactly where the idle stop is on your carbs.
But it may be worth a try if you're experiencing reluctant starting.
I said "only thing I will add" but two more: Once you get into a "this damn thing won't start mode" it gets hard to tell if it's starving for fuel or flooded... and the remedy for those are different. If thing it's flooded you can try cranking for a full 10 seconds with wide open throttle clear out the excess fuel. Then try a more conventional start procedure.
Finally.... If it's not cranking over fast enough it can have trouble starting regardless of everything else being right. IMO before starting extensive or expensive diagnostics of mags, plugs, fuel, carbs, etc., wise to consider replacing the battery first if not sure it's in robust condition. Other thing that helps having a robust cranking is keeping the battery on a float charger (cheap device) so when you step into the plane for first start of the day even if you haven't run it for a month the battery is fully topped up with charge.
