Postal puzzle follow up

Since most of the small post cards I sent out arrived at their destinations without a hitch, I went ahead and mailed another batch from the Stockton Post office. I put them in the drop box rather than taking them inside for hand stamping. A few got to their destinations but a group of 14, which the post office had banded together, came back to me with the "non mailable due to size stamp imprinted on a label attached to the top of the stack. Hmmm..... some make it, some don't. Most of these were mailed to locals, so my plan is to give them to each person when I see them. BTW none of the stamps were cancelled.