Note my original concern - the technique, unchanged, no longer seems to work. But previously did. If my technique was 'wrong' I would have always had starting issues.
Your technique is not wrong but it leaves little room for operational and environmental variation...lets us break down your technique before you start tearing your engine apart looking for a problem that may not be there:
Start procedure:
1/4" throttle
This increases the air volume, which increases the amount of fuel vapor required to light off, which subsequently requires more liquid fuel. How much more? Depends primarily on fuel temperature. The fact is if your engine idles well under normal operations, there is zero need to start a cold engine with 1/4" throttle. Especially if you have the mixture to idle cutoff (don't recommend), because you're just introducing more air, and no fuel.
What is 1/4" anyway? We really have no idea exactly what this equates to in fuel/air. It's really a technique we probably brought over from another aircraft type. However, the funny thing is, you can have the same engine hung on different aircraft types but the starting procedure in the POH is completely different. Why? There is no reason. However, you do know exactly what idle is on every engine because there's a physical stop and if things are adjusted correctly, the engine runs just great there.
Boost pump on about 5 seconds with mixture full rich
Obviously you're priming here, but you only have the throttle cracked at 1/4". You may need more...especially a cold start. 60 degrees may not seem warm but it's not, when you're trying to vaporize fuel. Remember, no fuel can't burn unless vaporized. Not even wood (a fuel) can burn. It must heat to the point a vapor is created to ignite. Don't believe me...google "How's does wood burn?" Jet-A...not technically flammable because it takes temperature >100F to create the required
vapor. Fuel is the same way...you need to get fuel
vapor, and cooler days and cold engines it takes more gas. How much more? That's the variable we can focus on because if you use the zero throttle technique every time, then you removed the air variable.
Mixture to idle cutoff
There is no reason to start this way, unless you're starting using the primer only which is not a normal technique on an opposed engine as it's injected into the jug rather that the blower on a radial, and it's a learned skill that has a lot of variability. We're trying to eliminate those in our starts.
Pump off
To stop the priming
Crank
On catching, Advance the mixture.
Try starting with the mixture full rich as others have suggested. Again, eliminating another variable! The timing and rate at which you reintroduce normally metered fuel should not be a concern to the pilot during a critical stage. Regardless of what you've been taught in fuel injected opposed engine ...think of the logic...you're dumping a bunch of fuel into a cylinder intake only to cut it off to the induction system. It doesn't make the remote amount of sense. Your incoming air should have the normal fuel vapor in it to sustain combustion.
I'm not saying there isn't something wrong here. Induction leak? Ignition? Clogged injector? Maybe, but if your engine runs normal otherwise than the likelihood of a weak component showing up during only a start is small and probably ignition related, but probably not if things light off normally. The easiest thing you can do is eliminate the marginal technique. We were all taught this technique and I used it for years before the enlightenment, but the starting struggles were always there until then. That's why there's so many opinions on starting, because it has been such a PITA since we were all taught it wrong.
I have a carb, so my technique is different but most certainly is at idle throttle every time to eliminate one of the starting variables.