Grafana 6.01 unable to send an alert to Slack channel

I’ve created Grafana dashboard that gathering data from Elasticsearch.
At the beginning the alert seems fine, but now it doesn’t works anymore.

Here’re the error messages

t=2020-05-28T14:37:50+0700 lvl=eror msg="Failed to send slack notification" logger=alerting.notifier.slack error="context deadline exceeded" webhook=de-data-alert
t=2020-05-28T14:37:50+0700 lvl=eror msg="failed to send notification" logger=alerting.notifier uid=Lqs2uB6Wz error="context deadline exceeded"

I tried with turn off image rendering and did search for the solution. However, no luck for me
also test by test button provided in Grafana --> working fine

Any idea ? Thanks