It may be helpful to try and consider your friends point of view.
1. Have they invited you in the past and you declined? Declining invitations most of the time discourages others from wanting to invite you
2. Do you and them talk outside natural conversation topics (for example, if you are friends in school, do you talk outside school about non-school related topics?)
3. Are you someone they perceive as a buzzkill or mood killer? Socially awkward folks are more likely to be excluded because others believe having them around would not be fun. There is basis to this because if you are anxious when hanging out, it's hard for you and others to genuinely enjoy the gathering.
4. Do you invite them to have fun on things that matter to you? It's a lot easier to invite someone if they invite you, because that signals to them you are interested and open to hanging out.