API running on Azure App Service Plan sleeps even with Always On enabled

I’m a novice with Azure, so any pointers on what to look at would be appreciated.

I have an API running on an Azure App Service (B1 plan), and have Always On set to On.

The API is accessed via API Management Service. If the API is not accessed for, maybe 5 minutes, it seems to go to sleep, as the next request to it will take up to 30 seconds to return. Subsequent requests respond virtually instantly.

As far as I can tell, it should be a case of simply setting Always On to On, but clearly this isn’t the case here. I’ve looked at the other responses I could find, but I either didn’t understand them, or they didn’t seem to apply to this situation.

Where can I look next?