Shake the tube and or improve the heat. Thats all you can do really.
For more info on what's going on chemically, research "Christmas Tree Bubble Lights". It's the same thing.
A $7 bubble light Christmas nightlight I bought starts bubbling on its own seconds after turning on. My Much more expensive MTH oil derrick will not bubble without shaking up the glass tube.
The MTH version of the postwar classic also has an annoyingly loud buzzing motor vs. the peaceful tic toc of the better design of ages ago.
My MTH oil derrick will be ripped out and replaced by one from a better era.