Sometimes the batteries don't connect with the compartment due to some gap between the battery and the receiver in the compartment. In this case, 

1. Check the batteries are making contact. The battery holders are quite tight in order to make sure that the battery doesn't fall out of the device. Check to see if there is a gap between the battery and the compartment. If there is a gap, wiggle the batteries, press from both ends so they make contact.

2. Remove the batteries, and re-insert it again.

3. If either of the steps doesn't work, you may need to get fresh batteries.

Please email hello@learnwithmochi.com if you are still facing issues.