People who are settled into a church and have fellowship together usually forget about newcomers and their sense of isolation since they are usually so busy within their own circle of friends. The church we felt connected to the most, years ago, had 250 people but it took us five years to finally feel like we were a part of the family there. It wasn't until we started helping out in the classes that people actually talked to us. They nodded, said hello and passed by to talk to their friends up to then. The next church had 400 people or so but it was filled with people that had been there since birth, it seemed, since they all knew each other and greeted/talked with one another and rarely said boo to newcomers.
I was in the nursery one day and overheard two ladies talk about how this church was the most loving and friendly and welcoming, etc... in the area and I interrupted and told them it wasn't. They said, "But of course, it is! We talk and laugh and mingle well past church hours!" I pointed out that to them, it felt friendly because they knew each other and talked amongst themselves but they rarely, if ever, truly included any newcomers. The next Sunday, these two ran around like crazy tapping all the new people on the shoulder and saying hello very enthusiastically. It was nice but that was about it.
Welcoming people is more than just a hello and a smile, even though it is good in & of itself, but it doesn't not create fellowship. Talking with and caring about others is what will make them welcome and loved. It is also part of what makes people want to stay and fills that part of one's heart that craves Christian fellowship.
I try to be that person, even though I am shy and fear rejection. We have been at a church for three years and still do not feel connected at all. Same thing here. Church claims at the pulpit how loving their church is, pat each other on the back, and continue to talk only amongst well known friends. I seek out new people and try to be the person I wished I could have as a newcomer myself. I get so little chance ---as a stay at home, homeschooling mom--- to be a witness or an arm of Christ to others that this is the least I could try to do. Despite this, I often leave the service afterwards without having been greeted by anyone, nor was anyone approachable to greet! They had their backs turned in to their little groups.
I hope you find some fellowship soon. Maybe a new church? If you are ever in Seattle, let me know