What is the Royal Mail doing ??? :(

Every December I post my greeting cards for my foreign friends. And sometimes, one or two return to me when my friend has moved to somewhere and I didn’t know it. Yes sometimes I can not help it. But this year! Today I got 4 greeting cards - MY greeting cards - I have posted at the beginning of Dec. All are to my Scottish friends. Of course they might have moved ... but about at least two of them, I’m sure they still live on the places where I have written its addresses on the envelopes. Actually I got the greeting cards from them some week ago! Then why couldn’t Scottish post office deliver my cards to them?? Is this because of the crazy Brexit problem??