The most common culprit is the 'Minimum Number of Responses per Student' setting on your activity.
By default this is set to 1, which means your students need to respond once to unlock the other students' responses.
Raising this number means they may need to respond up to 3 times before they can start commenting on their peers work.
This feature was designed to allow your students to provide multi-part, rather than single answers, before they can unlock the learning community and start commenting on the other students' work.
If you have any questions, or require further assistance, please click here to create a support ticket.