Failing to upgade a 108F-POE from 7.0.5 to 7.4.8

I am trying to upgrade the firmware of a 108F-POE switch from 7.0.5 to 7.4.8. In Fortigate interface, I can upload the firmware image, but the upgrade process does not start. Any idea what might be wrong? I tried other firmware versions as well (e.g 7.2) but no success