Plum should save for you every 4-5 days. If you had no automatic savings for more than 10 days please get in touch by typing 'chat to a human' or at firstname.lastname@example.org
This might be due to (1) a problem with the link between Plum and your bank account or (2) very low balance or (3) overdraft. If it's a link problem you can re-link your account in Messenger by typing 'Profile' and then swiping to ‘Bank account’.
In an overdraft situation, Plum will default to no savings. If you want to save while being in an overdraft type 'overdraft' so set yourself up.