Slack notification not allowed Grafana oncall

Hi. I’m trying to configure Grafana OnCall to send alerts to Slack, it’s almost working, however I am struggling to get it to mention people that are on rotation. The users in Grafana oncall have their slack account confirmed and the usernames match, however when the bot tries to mention the user in the slack channel it fails with ‘no allowed’. the message looks like this:

2m23s: failed to notify (, not allowed
2m28s: escalation started from the beginning

Slack permissons all appear to be correct, the bot has been added to the channel etc. Anyone seen thsis before?