Moving emails from inbox to separate mailboxes

I have been moving received emails from the inbox to separate mailboxes (NOT smart mailboxes), by clicking on the email in the inbox and moving the cursor to the specific mailbox and dropping the email into it; it disappears from the inbox. Now that no longer works -- the email in the inbox does not move with the cursor into the specific mailbox, but stays in the inbox. What has gone wrong and how do I fix it?