First of all, please make sure you have the latest version of the app.
Then check if there are any files download errors in Settings, and try starting download:
1. Go to project list
2. Open side-menu
3. Go to settings (press gear icon)
4. In settings, view go to the "Other" tab
5. You will see the section "Files on device"
6. Check if you see any errors in this section
7. Press start download
If that did not help then the issue might be caused by external apps on your device, such as antivirus software or auto-clearing cache apps. If you have any of those, try turning them off to see if that helps.
If those steps won't work, please make a screenshot of the error in Settings and contact our Support team from the app.