There is a natural 48 hour delay with the steps transferring to the Challenges. When you sync your Fitbit with your Fitbit account, your steps will sync with Go365 within the next 24-48 hours. After the next 48 hours they are added to the challenge.
If you average about 15,000 steps per day then there will most likely be a discrepancy until it catches up.
When all challenges are complete, there is a 72 hour window that allows for the remaining steps to be caught up and tallied for the challenge.