That's really weird. Muun is by default an onchain wallet. When you receive a lightning payment, it gets swapped onchain automatically for you. That means you don't have your own lightning node/channels, and should not be able to have inbound liquidity problems. Only the Muun node, which receives and forwards payments intended for you, can have liquidity problems.

Hope that makes sense. Did you get a message in the Muun UI about this, or how did this problem come up?