Hi @RussellFJ :
I had the same problem when I switch my Bulb account from my Monzo Bank sole-account to our Monzo joint-account. The direct debit just wasn’t taken for the next month (even though it’s normally taken around the 6th and I had changed it a couple of days after the previous months DD so plenty of time). It should come out next month (as mine did), but you can manually credit your account via https://account.bulb.co.uk Payments+Statements if you are concerned you don’t have enough credit already there.
The person at Bulb I emailed ( email@example.com ) seemed a bit unconcerned about the DD not being taken (even though I had received notification from my bank it had been setup in plenty of time) and just manually pushed a DD through so it doesn’t sound like an uncommon issue, but I do wish Bulb would have a look into it.