Ashley Seim - We haven't had any issues when it comes to having the ability to add attachments on a mobile device using Collector. For the most part, I believe most of our users are using an Android device. The issue we were having is we had users trying to add attachments from their laptops through ArcGIS Online and it just didn't seem to work. We could browse to the photo we wanted to attach, but the photo wouldn't attach for whatever reason.
For your issue...first of all, do you see the option to add an attachment in Collector? If memory serves me correct, once you create a feature, the ability to add an attachment is symbolized by a paper clip icon while filling out the attributes. If you don't see the paper clip icon, I might try re-publishing your service with feature access capability enabled.
If you do see the paper clip icon and are having issues getting the attachments to post, that could be a whole different issue...internet connectivity, cellular reception, etc...
