Similar thing happened to us last summer. We (DH and I, son #1, Son #2 with wife and 2 kids) all traveled to an out of town family wedding. Lovely affair, had a great time. There was a box for envelopes at the reception. Son #1 gave cash (in actual currency in a card), son #2 and family gave a gift card, DH and I gave a check for $1,000 in a card.
Our check was cashed right after their honeymoon. Our sons both got thank you notes a few months later. We didn't get one! So we waited a while longer. I could see the cashed check online and checked the endorsement, but I don't know what their actual signatures look like. Could this have gotten into someone else's hands? Not likely, but I guess it's possible.
We waited a couple more months and then messaged the bride. I didn't feel it was appropriate to ask her father, she's 30 years old! Just said we didn't get an acknowledgement and we wanted to make sure that it was them who got the money. She was shocked, she knew she wrote one to us and was very sorry we didn't get it. Within a few days we got a very apologetic note and pictures in the mail.
My guess is that it was lost in the mail. Maybe the original will show up one day tattered and dirty from being caught in USPS machinery. Or when the couple moves someday they will find it behind a desk or under a couch cushion. Whatever. We needed to know that they got the gift and we got our answer and the thank you note.