The consensus seems to be lack of cleanliness as the most likely cause. I will strip, inspect and clean the whole thing again, and this time I will remove and clean those two small cap stones. I have some Sambol Platina 1:20, which is an alcohol and ammonia based fluid, but had stopped using it because it's mixed with 20 parts water. It also leaves streaks and spots. I will use it this time and rinse in alcohol to remove the residues and water.
Thanks to everyone who tried to help so far. I'll report back when I've made some progress.