Depending on the Check-in flow defined by the admins, there are a couple of reasons why the visitor gets denied while checking in:
1) Visit denied due to smart rules associated with the advanced questions in check-in flow.
If the visitor answers the advanced questions asked during the check-in process with the responses that are defined in check-in flow to deny the visitors' entry, then on logging the responses, the visitor will be denied and the visit will get canceled.
The visitor will get an on-screen visit denial message.
The host will receive a notification of their visitor getting denied from checking-in.
2) Visit denied by rejecting the required legal document signing.
If your visitors refuse to sign a mandatory legal document that you have defined in the check-in flow, then as per the criteria, the visit will be considered as being declined by the visitor and will get canceled. Under such a case, the hosts will be notified of the declined status of the visit of their visitors.