Once the symptom has occurred, re-enter the 'set sntp server <server_IP>' command poll-retry.
To decrease the need for manual intervention each time the system is booted, decrease the specified polling interval from its default value to its minimum value: 'set sntp poll-interval <value>'.
In some conditions, examples below: The logs may report the basic message:
set sntp poll-Interval: 9 "seconds"
set snap poll-retry: 1 "second"
set sntp Timeout: 5 "seconds"
set sntp poll-interval 6 "seconds"
set sntp poll-retry 10 "seconds"
set sntp poll-timeout 5 "seconds"