Vibrate motor not working, neither is the replacement

So, a few days ago the vibrate component of my iPhone 4 stopped working. I no longer receive a vibration if I receive any sort of notification.

I figured the vibrate motor itself was likely broken, so I replaced it with one purchased from iFixit. Long story short, it still doesn't work. What's going on?

When I use the vibrate rocker, I still receive a popup on my phone telling me the phone has been switched from vibrate to sound notifications and vice versa, so it can't be the vibrate ring switch which is broken.

Any ideas? This is frustrating.