Vibration is the key to setting the proper reaction into motion.
Heat helps too. We also dialed up the zw for about 30 seconds and would lower it after things started happening.
Heat sink "grease" sounds like a good idea to help transfer heat.
I was recently researching bubble lights and tubes in general before this thread even started. All the bubble tube manufacturers have that same suggestion. Vibration or shaking.
If some folks had solved this, I'm sure the other tube makers would have caught on and changed things. But the MFG. suggestion to shake, hasn't changed in over 50 years .
My whole life I was twapping/flicking at tubes in derricks to get them started for a session. Gramps layouts always had at least one, usually 3+. Two or three quick twaps of a finger usually sets them into flowing right.
I'd never put one where I couldn't reach it easy.
Kids have smaller finger that's how I got the flicking job.
Maybe tapping with a pencil, or wood stick would work if you have big hands?
Never tried it, but 5 year olds don't have much power in their fingers either
The next time the same tube may need it again, or maybe not again for months.
It may bubble like mad, or just a little.
Or start big and die off(seldom). Or not start for while, or at all....or that day.
I wouldn't replace one unless it is dead and can't be helped more often, than not.
There are occasional lemons. But the tubes are not a perfectly operating mechanical device either. Chance can screw with the flow inside.
