Some people don't have their heart in the project. That's understandable if you're young and someone's asking you to work on a children's game but you'd rather be making Halo 6 or 7. Work ethic comes with time. What's unacceptable is when a team member is saving 'their' ideas for 'their' project. Let me help you with that.
There are 3 reasons that people hold back. First, they don't like the people they're working for and they don't want them to benefit financially from their work. Fine. Quit. There is no other answer.
A good team member doesn't make themselves into a weak link. Your integrity is a stake.
Next, you may feel that the project will be a success, but you won't get the money you deserve. That ties into the next issue, you're afraid you won't get credit for your ideas. The first thing you should do is to make sure you aren't over estimating your value to the team. Yes, your work may be the best thing in the project by far, still, if the project doesn't make money it doesn't matter. Next, it's fairly easy to prove that your ideas came from you. Stealing from a superstar is not good business--only losers steal from superstars. Last, you made a deal. Honor it.
Take my advise: if it's your work that causes a project to become a success, either the employer will try to keep you or you'll find a way to profit from it. Life is risk. Instead of listening to stories about rip off employers, do research on companies that go out of business. There's more of a chance that the project/business will fail, than there is of you being ripped off. Focus on your reputation--your talent will get you work, but your reputation will make you money.
So, don't hold back. Do the best job you can. Everyone has their own ideas that they believe will make them rich, or successful. However, if you never complete a successful project, proving that you can work with other people, chances are you'll never be in position to finish your own project.
Business is not putting on a suit and meeting with people in an office. Business is dealing with other people of all kinds, not just for money or profit but for many different reasons--mainly everyone want to get what they want. For some its money for others its power and for us its making games (um, and money).