Here are the most common causes:
Settings - By default, your camera is pre-set to send motion-based pictures instantly, but only wakes up once per day, usually around 11:40pm, to listen for incoming commands (Quick Pic and settings updates). This greatly prolongs your battery life. To change this and enable instant quick pic requests, you'll need to update the "Web Control" setting to Instant, either on the camera (SETUP mode > MENU button > 4G section) or using the app while the camera is turned on (Cell settings). TURNING WEB CONTROL TO INSTANT WILL DRAIN YOUR BATTERIES ABOUT 10X FASTER.
Firmware - upgrading your camera's firmware will ensure that your Quick Pics get sent. To update your camera's firmware, visit www.yellowstone.ai/firmware.
Connectivity Reset - Turn your data plan to Cancelled for between four and five minutes, then turn it back to your desired plan. Sometimes reinitiating a cellular connection can jump start your camera. This is an easy one to do remotely, possibly saving you a trip to your camera.