Keeping associate editor tasks moving is what keeps the publication process to schedule. When an associate editor’s task passes its due date, the editor-in-chief decides what happens next: extend the deadline, or hand the manuscript to a different associate editor.
Finding delayed tasks
The editor-in-chief can see every manuscript where an associate editor task has run past its due date, and notify the associate editors concerned. Open Review Tools in the left menu and click Delayed AE.

Extending the due date
Click the title of the manuscript. On the page that opens, extend the review due date by the number of days you want to allow.

Replacing the associate editor
Click the title of the manuscript, open the Peer Review Process tab, and replace the associate editor there.

Which route to take
Both actions are the editor-in-chief’s decision. Extending suits an associate editor who is engaged but held up; replacing suits one who is unresponsive or no longer available.