The amount of times my adhd has been a barrier between me and finishing and sending thank you letters... I always find them in a stack half written like a year and a half later. The gift recipient is thankful whether or not they have shown it openly about it yet. If it is bothering you, you should sit down with them and talk about how you are feeling. It may have just slipped their mind with the chaos of wedding and after. Give them an opportunity to be thankful after hearing you out