Children don’t listen

My kids are disrespectful to their grandparents. It’s not a thought. By social and family standards, they are. They ignore their grandparents when grandparents are talking to them. They yell at their grandparents. They don’t say, “Thank you” for gifts from grandparents, etc. I have been modeling polite and respectful behavior towards their grandparents and kids, explaining to them how to behave and how grandparents feel when they are treated that way. I have been very patient, understanding, and loving with my children. I don’t know where this attitude comes from. I need help with this.