AWS Lambda: The function is idle

It is related to VPC, please check this doc.

If your functions aren't active for a long period of time, Lambda reclaims its network interfaces, and the functions become Idle. To reactivate an idle function, invoke it. This invocation fails, and the function enters a Pending state again until a network interface is available.

You will need to invoke functions often using cloudwatch events regularly if you want your Lambda functions to stay alive & warm. Otherwise they will cool down as @Traycho Ivanov says.

Setup up cloudwatch events to invoke the Lambdas you need alive every so often, but how often is a bit debated it's not clear how AWS manages this and this could easily be changed into the future that your event is not frequent enough, or maybe it is too frequent costing you a couple more cents than you would otherwise want!

Instead of a Cloudwatch event, I would suggest using Provisioned Concurrency to keep a lambda(s) warm.