When a volunteer attempts to join an opportunity, or searches for an opportunity, there are quite a few variables that are being checked against the opportunity details to see if it is suitable for that volunteer.
These include their DBS and INDUCTION status, their age, gender, which group if any they belong to, the current number of volunteers already signed up for the opportunity, if the opportunity has been suspended, or if their DBS will be invalid by the time a session occurs.
It is usually due to the opportunity not being authorised or due to their DBS or induction status, so these are the first things that should be investigated.