Here's a few possibilities that I can think of with the limited info:

1. In your Animator, you most likely have multiple states, each one for each state of your character. If you simply have a delay in your character going from falling to idle, then it's possible that you have exit time on your transitions, which is causing it to have delay between states. You should do this for every transition in your animator graph. Example (The arrow properties in image should be, respectively, false, and zero.

2. Ensure you have a transition going from each state and back, not just one way. It's pretty simple to make a small mistake with a large graph like the player's animations, so double check that you have a transition back from falling to grounded.

3. Debug your isGrounded boolean. Perhaps it's just not actually working correctly, and it's gone unnoticed? Simply Debug.Log(isGrounded); will do the trick.

4. Alternatively, if none of these work, please expand on your issues, what is exactly happening, is the isGrounded variable working correctly? Are you stuck in falling, and what does your animator graph look like?