I used to send a second card after 60 days had passed and my card expired. Keeping track of that, I had less than a 50% success rate wit that registration number ever getting registered. Actually, I think it was around 37%. Checking whether or not they are active and when the last time they logged on is a good idea. Keep in mind that the recipient is not “missing” your card, as they do not know it is coming. Sending second cards gets expensive and time consuming, and Postcrossing assigns you a new number to send to after 60 days. For me, I just move on. Some cards either just don’t make it, or do make it and don’t get registered for whatever reason, Right now, during the pandemic, there are a lot of countries that have postal restrictions as well.
Here’s some perspective for you:
I can currently have 25 cards in transit, and I try to maintain that number full, meaning I try to keep 25 cards in transit at all times. Since 30 DEC of 2019 I have 11 expired cards, and have sent 350, so 3% expired. I suspect this is typical, but I don’t know, as i don’t have access to anyone else’s stats, but if 3% is typical, than you should realize that 3% of the fewer cards you may have sent as a newer postcrosser is going to seem like a lot; that is three out of ten cards. So it will feel like it improves over time as you’ve sent more cards. Also, some of those expired cards do eventually get registered.
Keep in mind that all of these numbers can be bit frangible. Some people ask others to register their cards even though they were never received, and some people agree to do that, for reasons that I do not understand. Also, if your card arrives after a year, it cannot be registered, even though it did eventually make it. I don’t understand that either.
Hope this helps.