Maybe there’s too much oil? Or there was some old oil from the time it was sitting in the shop’s inventory and it became sticky? Or maybe I’m doing some horizontal push when pressing the valves.

I think the first thing I can do is to clean it thoroughly, get all the oil of the valves and the casings (with warm water and soap), and re-apply oil. Second, I’ll try my best to not do any horizontal push when pressing down the valves. If it doesn’t work I’ll contact Yamaha.