Hi Tom,
I think I might know what´s going on.
The thing is, the 16 buttons represent the patterns, and the < ----> buttons the banks (1-12).
Not the other way around like you´ve described.
So the workflow is as such: select the bank (1-12) with the < ----> buttons and then hit the pattern button (1-16) where in the bank you want to store the pattern. Then it should be stored there.
As a sideinfo: when you press “Store” while being in the Store menu you exit the menu, saving the pattern in the currently selected pattern slot (blinking button).