1. Phoenix must be online when you receive the payment, as Ben said.
  2. Even if you have a channel with Acinq it doesn"t mean the payment will take the path directly between your node and Acinq. Usually, depending how the wallet app that send the payment is configured/designed, will use at least 3-4 hops and will try to take the cheapest route. That means that when you pay from your node will get out from the cheapest channel you have then find the shortest and cheapest route until Acinq. Then Acinq will forward it to your Phoenix.
Be happy that the payment is taking 3-4 hops, that means more privacy