No worries, we have lots of ways to get the videos.
First, figure out if they were shot on the app or imported from your camera roll. If imported from your camera roll, were they texted or emailed to you? Sometimes when files are too small, they will upload but not be added to the project. Texted videos are too small to be in an edited video. You'd look like a lego mini-figure. :)
If the video was texted, contact the person who originally filmed it, so they can import and upload through the app.
Next, if it was shot on the app itself, it may still be uploading in the background or processing. Check the gallery on your app to see if it uploaded, or give it a day or two. Unfortunately, iOS cuts off uploads every time the phone goes to sleep. The WeddingMix app will resume the upload automatically, but it may take a while.