Have you checked the cable? The most common cause of sticking lever I’ve seen is a kinked cable inside the shifter. Try pulling the cable out. Cables can occasionally become kinked 1cm from the nipple in SRAM shifters. I’ve never figured out why though. They still shift but but the lever doesn’t return.
The upgraded replacements sound like the next option.
You could dismantle and check the spring on the lever. I’ve never pulled apart an X9 but I have pulled apart a few other SRAM shifters. They all seem to have a v-shaped spring. This can slip out of it’s grove. Bending it back a little and then refitting fixes the problem for a few months. If you do pull it apart I recommend you use good quality screwdrivers and press them fully into the screw heads – they round if you’re not careful. Take the housing apart very slowly – you want to open it so the springs all remain seated. If you open it the wrong way up everything pings off over the kitchen and you’ll spend hours trying to work out how it goes back together!