I have 3 yong heng compressors. Well two currently and one that will arrive in a few days. One that I use and two will still be new in their original package (impulse purchase lol). Anyway I have found that if pressure is not building at least in my situation it was always a leak in the system somewhere. It is very easy to miss a leak while running, and when you use a deadhead, it could still be leaking but the one way valve that's built into them will prevent you from seeing a few other places where it could leak. I have found leaks at the bonded washer coming from the top where the high pressure starts, also found leaks at the high pressure where the tubes connect to the piston assembly, leaks also found around the pressure gauge, another very annoying leak that i have found is at the burst disk. There is a small washer that presses against the burst disk that is supposed to hold pressure. What i have seen is that that washer can develop hairline cracks almost invisible to the naked eye and that will cause a leak. Looking at the burst disk being intact you will think thats not the issue and it may not be, because its the small washer in between the burst disk and the screw holding it in place. Every single time for me anyway when it seemed that it was not building pressure or seemed very slow it was always a leak somewhere and it was not actually worn out.
