I sent mine off soon after the swap scheme was introduced as I thought it would be busy nearer the cut off.
Gave them quite a while, but heard nothing. Rang up and they checked, said mine had never been received. I had taken copies of what I had sent, and she asked for the details and sent me the replacements.
According to the website, they didn’t have to do that, so was grateful. But my cynical side wondered if unscrupulous delivery people could just keep the special envelopes with the stamps in and claim themselves, thus not showing as being received from oneself.