I have been brought up to send Thank You cards or messages when receiving a gift for birthdays etc etc. My OH Nephew was married back in March, we put £150 into his Honeymoon fund (they didn't want pressies). This is a new concept, I think nowdays. We have had no Thank You or any acknowledgement, not even at the wedding.
I don't expect a card but even just a text to acknowledge a gift is appreciated. I know a few people who don't do that, just take the gift and ignore it. A one off forgetting is forgivable but repeat offenders get all the crap stuff I don't want to give anyone else now.
In view of your generosity, I think you are entitled to be peeved!
It may be that the thank-you notes are late in being mailed, but that again is a discourtesy to guests who have been kind enough to offer a gift.
Dependng how much you want to pursue this you / OH couild ask if they got your gift - which should produce the courtesy of a deep blush and a thank you - otherwise you will need to write it off to simple ignorance and bad manners.
I have always made my Son from a young age write a Thank You Card or send a drawing (when he was very small) as a thank you for birthday/christmas money or present.
I think Sqad is right, manners seem to be a thing of the past.