Merchant Lightning Capacity

A local cafe owned by a lovely Vietnamese woman advertises they accept both Bitcoin and Lightning payments. I tried to pay for a coffee today but we did not have any success. She was using Muun wallet on her phone to facilitate the transaction, and I suspect it's to do with incoming transaction capacity on her end. Using both Blue Wallet and Muun from my end saw the same result. I doubt many transactions in BTC happen here at all, and she indicated she almost only used the Bitcoin network in the past which is growing increasingly unfeasible with all this jpeg action.

What advice should I give on how she can increase this capacity? I know that Muun uses Submarine swaps which might be relevant, but I'm unsure of the technical implications of this if there are any.

Thank you!