"Sweeping" just creates a standard Bitcoin transaction, that moves coins from one private key to another. It is basically the same as creating a transaction in Electrum that sends the coins from Electrum Address to Coinomi Address... but you're creating the transaction within Coinomi and using the private key from Electrum to sign it.

From your message... you seem to have:

A. Created a wallet in Coinomi using the seed mnemonic from Mycelium. This wallet has BTC and BCH addresses setup.
B. Attempted to sweep BCH from an Electrum address/private key to one of the BCH addresses in your Coinomi wallet.

Did you get a transaction ID from Coinomi? Have you looked at the Electrum address on a BCH block explorer like blockchair or blockdozer to find the transaction and see what BCH address the BCH was sent to? and checked that that BCH address is actually in your Coinomi wallet?