I just double checked in my Settings --> Sound --> Volume menu and I have nothing like that. As far as I know, all volume controls are independent, with the exception that you can tie Notification volume to Ringtone volume. Maybe double check in this menu that your Ringtone volume is all the way up if you haven't already. I'm not saying you didn't turn it all the way up using the side buttons, but, even if you did, can't hurt to double check this menu is displaying it as such.
And, when you say you are having a problem hearing it, do you mean it's just not that loud when you are listening to music in your car, or, when it's really quiet in whatever room you are in. Because if the volume is all the way up and you can't hear it in a quiet room, there is definitely something going on, haha.
Have you tried using another ringtone to make sure it's not just that specific ringtone that is the problem? Are any of the other volume functions of the phone giving you issues? What about music? Or anything that is considered a Notification? If your Notifications are low, go into Settings --> Sound --> Volume and decheck Use incoming call volume for notifications and then set the notification volume all the way up. Then send yourself an email to see if the notification volume is audible.
If that doesn't work, I dunno what I'd do. Depending on how much you have done with the phone and what you'd lose, you could always factory reset. Have you installed any apps? If so, could be an app conflicting. Or were you having issues with the volume right from the start? If you didn't experience the issue until after you had installed apps, I'd suggest uninstalling them all (depending on how many you've installed) and see if the volume issue is fixed. If it does, you can reinstall apps one at a time to see what app is the problem.