Hi Bella,
Okay, lets do a little brainstorming here.
The volume/power buttons are connected via a small flex cable going to the circuit board on the left side of the battery (right side if you're looking at it from the back with the case opened up. Given that you said everything worked after replacing the left joystick, let's assume everything was good at that point.
Now to do the right joystick you have to remove the motherboard. The only connection between the motherboard and the daughter card is that somewhat fat flex cable that starts right above the battery connector and goes along the bottom of the battery to the left side daughter card. You do have to disconnect that to remove the motherboard in order to access the right joystick. so my guess is that's where the trouble lies. You'll need to check the cable itself for any kind of damage; it will probably be small if the power button is the only thing being affected. Look along the length of the cable, but especially the contacts at the end where it plugs into the motherboard.
In addition, it's possible one of the pins on the motherboard connector is bent out of place or broken. A bent pin may be able to be straightened with a pair of fine tip tweezers, but a broken one will require the replacement of that connector, a difficult job for an experienced DIY-er, but should be relatively straight-forward for a repair shop.
Look at everything I've mentioned with a magnifying glass and a bright light. If you don't find a problem, go ahead and clean the connector and both ends of the flex cable with 90% or higher isopropyl alcohol (don't use the 70% rubbing alcohol we all have around the house) and reassemble. Check if that's made any difference and let us know what you find.
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