Hello Sandip,
Is this a site-to-site VPN? If yes, do you have access to both endpoints of the VPN tunnel? 1 hour is the default timeout for phase2 so I am wondering if one side might go down while the remote end still have phase2 up. In this case, the tunnel would come up at the next rekey.
You could check if isakmp keepalives are enabled and turn them on as a first step (periodic keepalives) to see if it helps.
Warm Regards,
Rose