It is likely to be one of three things:

1. Your 24 word seed phrase is incorrect
2. Your 24 word seed phrase is correct, but you also used a wallet passphrase with your Ledger Nano (is it the "S" model or the original Nano?)
3. Your 24 word seed mnemonic is correct, but you're using the wrong derivation path

I don't think it is likely to be #3, as I'm fairly sure that Mycelium and Ledger both use BIP44... so it should be using the correct derivation path of m/44'/0'/0'