Why some churches gain members and others lose them?

It depends on a lot of things. Sometimes the neighborhood changes and the people who move into the area are not interested in that particular denomination. Sometimes the membership begins to age and they don't attract young people.

Just because a church is busting at the seams doesn't necessarily mean it's better than the others. Mega churches attract members by entertaining them. Sometimes they can be a mile wide and an inch deep.

Because well, some are better convincing than others. Even though I can fairly say even though I'm an atheist the church I would probably join is the protestant church here in Sweden, as it is very free and I even had a wedding where we said "we don't need to speak about god and so on" and the priest didn't even look like he cared. He just said like "Okay, sure! We get a lot of people here so we have to accept all" and so on. These free churches usually lose members since humans have a capability of being brainwashed, the heavily catholic church probably gains more members than the free protestant church in let's say Norway. I am saying this because the brainwashing and heavy christian that are in poor countries get more, and the ones in more wealthy countries get less. And when I say "wealthy" I don't mean like the US, I mean each citizen has a higher amount of money and can live better and so on.

some Churches think they can control you. The ones that dont try to control you are the ones that grow.