SSH SFTP Deployment Password not remembered for some sites.

I have several projects that deploy to different sites on the same (Plesk managed) server. As far as I can remember, they were all set up the same way, in both PHPStorm and on the server side.

I use SSH SFTP for deployement and each has an SSH configuration using Key Pair. The SSH certificate has a passphrase, and this is stored without a problem.

For some sites, PHPStorm asks for the site password once and then never again. Yet others, which seem to be configured exactly the same, always ask for the password when the project is opened. Most of the time it will accept it and it will be fine for the rest of the session. However, sometimes it will just sit with the cursor spinning and not connect until I reboot the IDE.

Anyone got any ideas?