This is often due to a poor internet connection or a large video file. Try switching from cellular to Wi-Fi.
SmugMug does not have a "background sync" that runs 24/7 like Google Photos due to iOS restrictions. However, the has a robust "Auto Camera Roll Upload" feature that works reliably when the app is open in the background. smugmug auto upload iphone
If you shoot with apps like Halide or Lightroom, your iPhone saves DNG (RAW) files. SmugMug supports RAW uploads. RAW files are 25MB+ each. Ensure Wi-Fi Only is enabled, or you will destroy your data cap. Also, note that SmugMug displays RAW files as thumbnails; you need to click "Download Original" to edit them in Lightroom or Photoshop. This is often due to a poor internet
If your photos are not uploading automatically, check the following: However, the has a robust "Auto Camera Roll