suggestions for plasma globe that is slow to turn on?

I own several plasma globe type devices basically similar to this:
https://www.arborsci.com/cdn/shop/products/PlasmaGlobe8inch1500x1000.jpg?v=1652122477
I also have a dome, a disc, and another device resembling a spiral glass tube.
Several years ago, I took a look inside all of them and they basically all have the same type of circuit: a single transistor driven small flyback in the 3-8 KV range coupling to the glass tube or globe.
One thing that sort of bothers me about my devices is that, over time, a couple of them have become slow to turn on, especially after sitting for a period of time, but not in every case: one of my plasma domes is difficult to turn on all the time.
What I mean by the aforementioned is that, once the device switch is activated, the glass will remain dark for a few seconds before it starts to show "plasma".  In the case of the plasma dome, there are two controls on the front of it controlling intensity and frequency that I have to fiddle with for maybe 30 sec sometimes before the globe will activate.
My question is, what causes this behavior and how do I rectify it? Thanks.