Why don't I see the spam folder in my email client?
One common reason this would occur is due to the use of POP3 instead of IMAP. The POP3 protocol only downloads email from the Inbox and sent folders. If you are experiencing this issue and configuring your client to use POP3, this issue can often be resolved by configuring the client to use IMAP instead.
Another reason this might occur would be that the spam folder is not a subscribed folder in the email account. In Roundcube, you can subscribe/unsubscribe to folders via "Settings > Folders"