Its highly probably....it doesn't matter. The VTR is likely low because your items aren't getting a delivery scan. You don't state what Royal Mail service you are using, but if its Royal Mail 48 - you'll never get over 80% VTR - but Amazon validate unscanned Royal Mail parcels.

The exception, which may be an issue is letter size post - for this, you HAVE to use Amazon Buy Shipping to stop it impacting VTR.

As per VTR FAQs..

Always make sure that you or your integrator provides the correct name of your carrier, the delivery service used and the tracking or package ID. You may receive a Valid Tracking Rate warning as a result of low scan rate. However, we will validate unscanned tracking or package IDs with the integrated carrier before suspending your offers in the affected category. Your offers will not be suspended if you have provided valid IDs, including for all parcels that have not been scanned by the carrier. This includes delivery confirmation-only methods such as Royal Mail 24, Royal Mail 48, Royal Mail first class or Royal Mail second class.

Note: Note that shipments with Royal Mail 1st and 2nd class letter stamps will only be exempted if you buy them through Amazon Buy Shipping, which is available through Amazon Buy Shipping at no additional cost. For further information, see our VTR help page here and visit our Buy Shipping page here.