Missing bills

I have been in touch a couple of times as I have built up a £400 credit. You say that there are two bills missing despite me giving readings. Feb and March have not been generated.
I get the impression that this is quite common but no-one seems to be able to tell me why and when it will be corrected.
My electric meter also fluctuates between 88888888 and a normal reading which you say indicates an error but again I have not been able to get this fixed.
Please can you sort this out as I need to understand my usage and costs for the winter.
Thank you