What does my Thai girlfriend mean when she says, "Why you do but not do when you say you do but don’t"?

The phrase from your girlfriend suggests that she feels disappointed or frustrated because you are not following through on promises or commitments you've made. Many comments offer interpretations like "You said you would do something, but why aren't you doing it?" or emphasize the importance of keeping your word in relationships. This seems to be a universal issue in relationships, highlighting the need for clear communication and follow-through on commitments.