Here's a trick:

Typically-- the glow plug will be mounted in the 6 o'clock position when it's inverted. I.E.--pointing straight down.

Try remounting the engine just a little off center--so the the plug is at maybe 6:30 o'clock or 7 o'clock position.

Or go the other way and mount the engine with the plug at the 5:30 o'clock or 5 o'clock position.

Just getting the plug from being on the exact bottom will sometimes help. It keeps fuel from pooling directly on the element and cooling it down.

If a little fuel pools up in the head--but NOT directly on the plug--it can sometimes help with those low-throttle flameouts.