We recently had an issue where icons were missing for all users in OWA, I removed the backend and front end extensions and this seems to fix the issue. However I have one particular user when logged in OWA icons are not there, I can see the emails archived for this user in the mailbox.
it can make a difference because when EV registers the OWA forms, it will register them against the system mailbox for each storage group, if the system mailbox doesn't exist, then the deployment will fail for that one,
So lets say you have
Storage Group 1
Storage Group 2
Storage Group 3
All but SG3 have the system mailbox, meaning users can see the icons and buttons unless they're located in SG3
If you look in the OWA set up logs in the EV install directory for the backends you will see if the setup finished with errors or not and you can also see if it failed to find the system mailbox.
Also in the same directory is the OWA Configuration INI files to which you can set the logging to 2 which will output the logs in to the \logging directory