Such an issue may happen if there's not enough memory space on your device. Also we recommend checking whether Prequel has access to your Photos.
To do that go to Settings > Privacy > Photos > Prequel > Read and Write on your device.
If you need any assistance, feel free to contact us! You can do it here in the app via Contact us button or email us at support@prequelapp.com.
We're always happy to help :)