File early. The window is shorter than people think and it starts at the transaction. A fortnight in on UPS and it lines up.

Careful with attribution. You changed the timing, the dose and the season. That is three variables.

You are not imagining it and you are not being difficult. Document it and take the document in. Not on a billing error but the pattern is familiar.

Test one thing at a time. It feels slower and it is the only way to learn anything.

Written confirmation email after every phone call. Cheapest insurance there is.