If a direct debit has a variable amount, and it’s setup to be taken on a regular schedule - when it doesn’t get collected in a month it means my current spend for that DD is zero, not the last non-zero amount that was collected many moons ago 
My credit card DD comes out near the end of my summary period, and because DDs are pushed magically into committed spending with no options to skip/remove or adjust/exclude individual predictions it means summary constantly allocates money for me to buy a new laptop each month because that happens to be the last time my card DD triggered 4-5 months ago and it’s not been used since.
It takes around 8-10 days after the normal collect date before summary gives up expecting it to come, by which time my summary period is ready to reset and it starts off all over again in a new period 
The only way I can think to break the cycle and get summary out of the red permanently is to use my card again for something small and let the DD collect this month to give it a new figure to work from - which isn’t great.
Some DDs will naturally skip a month or two (council tax etc), so probably needs some thought - but an obvious credit card DD is always going to have wildly differing amounts taken and can badly break predictions as it’s often designed for sporadic high-value purchases, but no activity for 4-5 months should be enough to work out it was a one off
