When I use /smartlock/{smartlockId}/action to unlock it doesn’t work in about 10% of all cases.
According to my logs, the POST request returns the HTTP status code 204 reliably every time (which should be an indicator for successfully triggering the action according to https://api.nuki.io/#!/Smartlock/post_0).
But it does not always unlock the door and there is also nothing in the Nuki logs in those cases (https://web.nuki.io/en/#/admin/logs).
Does the 204 status code really mean that your API received and acknowledged a command? If so, it would be a strong indicator that actions are not executed reliably.