Hi, if this is before you choose your LI then it's because you haven't earned enough approval points with them to qualify for their route. If it's the V route later in the game, it's that you have not earned enough approval points with him by that point. Hope this helps! :)
You have to pick the choices that they would agree with and avoid saying things that would frustrate them! It's different for every character so you have to take a different approach with each of them based on what they like ^^ is there any particular part you are having an issue with? Have you checked your MCs sexuality matches the LIs gender?