This is a very common problem. In general who came up with the majority of the game ideas and concepts (does everyone feel some form of ownership to the game concept)? Is there any team lead or is everyone just off doing their own thing? Does everyone have a specific task to focus on with reasonable deadlines? Are you working on more smaller and manageable tasks first? Do you have weekly or frequent meetings where everyone gives updates and shows off what they've done? Is there a reward system in place?
I'm not sure what went on for a month and a half but if you had a bunch of people sitting around a table spouting ideas while you were working on majority of the stuff then it isn't going to get better. I've seen this happen over and over again, you get individuals all sparked up and ideas are flowing in non-stop for a period of time, then when actual work needs to be done people shut down. The fact is development isn't always fun and you need some form of a structure in place with someone at the helm leading the way. You have to find a balance between hard work and still having fun because this isn't a paid project. Discipline is key here, not motivation.
On the subject of “motivation”… Motivation follows action and not the other way around. You need to focus on getting things done and work towards everyone on your team having a feeling of accomplishment as they're contributing and moving the project forward then motivation will follow, but you have to keep the momentum up.
Take your common goal, break it up into sub goals and set targets. Make sure everyone has a purpose. When targets are reached make sure to reward yourselves with a night out together or some event, dinner or whatever - agree to some rewards. Train yourselves that if you complete (x) goals the entire team will share into some common reward.
Break down:
- Have structure, leadership, purpose, and accountability
- Set small goals that are manageable and make sure everyone knows what they're doing
- When goals are completed set rewards that the entire team shares in
- Work, work, work… Keep up the momentum because with enough time away or lost focus the entire project can go from extremely productive to abandoned
Beyond that… if you're doing everything right and people are not contributing their share then I would remove them and find new members. Make it known that the club is for game development and not a brainstorming club. Finding devoted people who will actually put in the blood, sweat, and tears isn't easy - especially when it's unpaid. Even people working on their own solo projects tend to hype up at the start and flop over when work needs to be done.
Best wishes.