Depending on fuel grade, fuel delivery to combustion chamber and a few other subtleties, I have noticed on various occasions that a plug that looks good when lit out of the engine may still not have the right heat once installed back in the engine. Could be you're getting enough heat to make it fart, but just a bit short of enough heat to make it catch and run. Could be the grade of plug itself is not what your engine/fuel want.

I have a Hobbico starter box with a glow driver that permits adjustment of glow heat over a considerable range, and have found some plugs/fuel mixes like to be at the top of the range to start easily, while others at the top of the range pre-ignite, backfire or start backwards, and want a lower heat to start easily. Some plugs are rated for 1.2 volts, some for 1.5, maybe others in between.

Lots of stuff has been written about selection of glow plug, and sometimes even the engine manufacturer's manual gives a plug recommendation that is not optimal in practice. Again, the grade of fuel you're using has an influence as well.

If you're getting farts but no starts, the first thing I'd try is either a bit more voltage to the plug if you have a means of adjusting that, or a freshly charged igniter, or substitute a hotter plug. For easy starts you need the right combination of plug type, fuel/air mix, grade of fuel, voltage delivered to the plug, and maybe another factor or two that escapes my notice. When you get the right combination, note on your engine box or some other permanent record what the components are for easy starting and clean running.

If it's a popular engine, other guys at the field can maybe tell you what combination works for them. I wish I could make it simpler for you, but unless you can get a good working combination from someone with the same engine, you just have to experiment.

The fact of it being a new engine should not matter much. I've had plenty of brand new engines start easily on the first flip.

Also, an electric starter will often help give a good fuel/air mix, at least eliminating that possible problem. I presume you are priming a cold engine? I normally start an unfamiliar engine at about 1/4 throttle opening, but sometimes find I need to go to half throttle to get an easy start. Once I get the thing figured out, it should start easily at idle or just a couple clicks above idle.

We need to hear from someone who has an easy starting ST51 what plug and what fuel they're using. Supertigers in my experience are not hard to start. I had an ST90 some years back that liked a hot plug (K&B 1L). One pretty good glow flyer at our field puts 1L's on all his engines. Others swear by OS 'F'.