If the family dynamics are not fine, then their lack of appreciation may be a way of expressing their anger or hostility toward you. Parents are often taken for granted by their kids. If you and your kids get along great, then they probably don't realize that a thank you to a parent is important. You need to sit down and have a serious talk about this with them. If they also ignore other people who give them gifts, then tell them that their lack of appreciation makes them appear selfish and ungrateful.
People judge others by their actions.
Let them know that there will be a consequence to their not appreciating the gifts that you give. Decide what the consequence will be before you have the talk. For example, you will give a monetary contribution to a charity in their name instead of a direct gift to them. It's important that you follow through and really do something other than give them a gift. Always call or send a card to let them know you're thinking about them, and let them know about the donation in their name.