SFP Prime Metrics dropping each week

We are having a reoccurring issue with out seller fulfilled prime rates dropping each week and we cannot understand why. We have checked our metrics and most the orders on the form having matching Ship by and actual shipping dates. We had thought that it could be due to the OTDR but after looking at the forum have found that there is no penalty for this, please correct us if we are wrong.
We pack our orders on a Saturday on the weekends and they are collected by Amazon shipping on a Sunday as we are not allowed a Saturday collection. This automatically pushes back the shipping date and delivery date by one day. We have explained this to Amazon and they have said it should not affect the metrics but the bulk of the report every week is our Saturday orders.
We have asked for help to enquire if we are doing anything wrong and seem to get passed around to different people and never get a definitive answer.
Has anybody else gone through this or can recommend anything that we might be missing.
We would greatly appreciate any help that anyone may have to offer.