new volume flex won't work as properly

Sometimes the replacement button assemblies have to be "tuned" to work properly. I do a lot of proximity flex cable replacements....Which has the power button on it. And usually have to back one or both screws out slightly to get the button to click and function properly and feel correct. Sounds like you have tried to fiddle with it but see if that does anything for it.

Also, you may want to check behind the metal button and on the cable itself....the button on the flex cable should have a little black dot in the middle of it. This helps it to make contact with the metal button and click properly. If its not there, you can try putting a small piece of tape in the center...Just to increase the spacing and hopefully get it working right. If it were me I would simply replace the cable if the black dot on the button is missing.

Hope this helps!