Your email program may pick up messages before Spamdrain get to see them. To solve this it normally helps to increase the interval your email programs check for new messages and disable Push (sometimes called IDLE). Using IMAP instead of POP3 is also recommended as IMAP is a more modern protocol.
To update these settings in your iOS device follow the steps below:
Open the Settings app
Go to Mail settings
Go to Accounts
Go to Fetch New Data
Turn off Push and change to fetch every 15 minutes.