The first thing that comes to mind is that their channels might not be balanced, I'm assuming you are using a 3rd party LN node.

I had issues with some LN wallet I tried in the past but forgot the name, and it was something similar to what you are saying, I had sats in my wallet but I couldn't move them out, I reached out to the maintainers and they said it was just a matter of time for them to rebalance the channels and then eventually I was able to.

Some wallets might also allow you to "loop out" meaning you would receive those sats on the Bitcoin network, at the cost of a fee which depends on the node runner.