Usually, this is for one of two reasons.
First being that the bonus may have expired.
To view the active bonuses and their relevant expiry dates on your account right now, head to:
Bonus History in My Account for Casino Bonuses of Free Spins
Make sure to use it before you lose it!
The other reason could be that you have made a withdrawal.
When you do this, you automatically forfeit any bonus that in your account.
You will receive a warning before you do this, so please pay attention to your screen.