IHD not showing correct usage?

I have emailed Bulb’s complaints team but have had no response…I wondered if anyone else’s IHD shows lower readings having shown higher ones earlier in the day?
For example at tea time it said our usage was £5.15 (it’s been a tumble dryer kind of day!) but later on it said the usage was £3.56.
How is that possible?
My gas meter is also not sending readings and there is no option to submit a reading myself now we have smart meters.