Currently I have 3 items posted this week using RM 48 hour tracked, showing as delivered on the RM website but no updates on Ebay - the system knows they have been sent because I have the option to 'mark as not dispatched' but no updates on the tracking.
However, this is not a Simple Delivery issue as none of the things that were posted were sent using SD. A skirt sent on the same day (with SD) has been marked as delivered.
I am not sure if the issue lies with Royal mail or Ebay but it is unconnected to Simple Delivery,

"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)