It’s a little weird it’s all of them not just one or two, so maybe someone has a better idea, but the symptoms sound like it’s a gas cylinder problem. Afaik they sometimes get a little dust and dirt in the seals and don’t seal off proper any more. But since it’s all of them the gas cylinders might just be cheap or not suited for the task.

I don’t know if you can repair the cylinders themselves, I’d be a little a lotta careful, since those are built to handle some weight they can be under a fair amount of pressure. So disassembling one of the cylinders might will be dangerous, if you don’t know what you’re doing

There are several different sizes of those cylinders, so you’d need to measure one of them before ordering. I’d just look up what measurements are listed for those cylinders online and then disassemble one stool and take the same measurements. Other than that they should be replaceable fairly easy.

That’s the knowledge I accuired online when my beloved office chair started to develop the same issue, but since it still functions most of the time I never got around to actually disassemble it, so take everything I said with a grain of salt and maybe get a second opinion. Good luck!

edit: after looking at the other comment I emphasized the dangers those cylinders can inherit, I still think that replacing them will be fairly easy since the replacement parts are offered online alongside instructions how to