For VAT direct debits, I believe HMRC will usually start the collection on the filing/payment due date (7th), but as these take a few days to clear, you usually won’t see the money collected until around the 10th, depending on weekends. This is confirmed on HMRC’s website.
If it’s your first time paying VAT via DD then I would recommend double checking your online banking to make sure it’s all set up. Depending on who you bank with, you may be able to view pending payments.
All the best,