Direct Debit request is sent based on outstanding balance. You can view it on latest bill. If there is anything like for example:
Balance brought forward £30
Then it means that payment was taken during last billing period. If line with payment is missing - then it means that we did not receive any payment last month. If you your bill is higher than usual - there is list of every charge on your bill where you can check if everything is correct.
Usually it is caused by additional call charges, change of billing date or changing product.