Inconsistency in Voicemail store on phone notification

I am using asterisk version 13.1.0 on Centos 6 and Polycom VVX phones. I have been trying to leave and retrieve voicemails and have enabled store voicemails. It stores the message on the server but it is not consistent in notifying it to the phone.
It is behaving randomly for different users. Sometimes it alerts 1 message waiting and then suddenly it dispairs and some times it doesn’t notify at all. :frowning:
Could someone please let me know if I am missing any configuration or is it a bug in asterisk 13?