Sorry to hear you're having trouble with Instant Upload. Have you enabled location services? Have you updated the app since installing it a few months ago? As a first step, I'd recommend updating to the newest version of the app. Then try opening the updated app and try uploading all the photos on your camera roll again. Keep in mind that you may need to keep the Google+ app open during this process, as iOS only allows apps to run in the background for up to 10 minutes.
Make sure you enable location services for Google+; from our Help Center: "Apple requires location services to be enabled for an application to access the photos on the device’s camera roll. So, you have to have location services turned on to use Instant Upload. Enable location services in your device settings, under Location services > Google+."