It has happened to me plenty of times and there is usually a reason for not being invited. The reasons have been that I was too boring and that they thought I would just sit there and do nothing, another reason was that I would get drunk and act stupid. Some people also don't like others who are always trying to hit on all the females. Your friends reasons might be different than these examples and the only way to find out is by asking, and like jeffthechef said "You could be improving yourself". In my case, I already knew what the reasons where but I used to say to myself "This is the way I am and I'm not going to change for anyone", well this attitude didn't get me anywhere until I decided to change some things that I knew I was doing wrong. Also, make more friends, the tactics that we learn in here can also be used to meet friends, by just being the charming outgoing person that people like to be around, guys notice that too and think to themselves "This is a cool guy, I would like to hang out with him, look at him he is not afraid to talk to girls, maybe he can help me out" and that is how you make new friends that will look up to you and consider you the alpha male and have great respect for you. These are just my experiences and my 2 cents.