Sorry to say that very few young people have been raised with any knowledge of etiquette at all. Everyone is on the computer and they don't know how to write thank you notes! If they don't have your email, they don't do a thing.
They are SO concentrated on the gift registry as if your gift is your price of admission to the event of the century. Usually the parents do the invitation lists and half the time the addresses are computer generated, so no one ever puts pen to paper.
There is a general "one year" time frame for giving a wedding gift, and a lot of people have a "one year" window for thank yous. But it's unlikely to happen if they aren't done in the first 3 months, frankly.
If you mailed the gift, you could ask if they received it. If you took it to the wedding, that's a problem sometimes - someone has to be responsible for hauling everything out of there and getting things home. Sometimes cards get separated from gifts. Our friends' daughter had all of her "envelopes" stolen by someone at the wedding - either a guest or a waiter. So all her cash is gone. But the parents contacted all the guests and asked them, explaining that if they didn't get a thank you, that's why. However, in your case, I would say it's unlikely that this happened in 2 different weddings. I never take gifts to weddings anyway because of the hassle it causes the recipients, and now I always ship things with a return receipt and insurance.