What should I do to a team member who is not satisfied in the team.

I'm not a cofounder of the team, but I could be one of the founding members. I've built the MVP product, product team, and dev team. I'm the leader of the product team and dev team. In the dev team, one software engineer looks very dissatisfied with the company, but he doesn't say anything about it although I have 1-on-1 every two weeks. Due to the budget, we hired a lot of juniors without seniors. These days, the atmosphere of the team and company is going wrong, but I don't know how to fix it.