I have four RPis. Two are 4s, one is a 3B+ and one is a 3. The RPi3 runs Stretch and the others run Buster. All have VNC enabled. I also have a couple of Windows 10 PCs. On my primary PC, I can connect to any RPi with VNC Viewer after a fresh boot of the PC. Later if I open the VNC Viewer and click on one of the RPis, it won't open the connect window. If I reboot, I can make a connection for a few minutes. Later it will quit opening the connect window. On another PC in another room, it works all the time. My current workaround is to connect to the other PC via TeamViewer and have that PC connect using the VNC Viewer.