First, do you know that the issuance transaction was successful? Did it return a transaction ID and can you see the asset in listassets?
Second, did you import this address after the issuance was performed, or before? If after, did you pass true for the rescan parameter?
Finally, you may have to wait for the transaction to be confirmed, unless you pass 0 for the minconf parameter of getaddressbalances.
If none of these suggestions help, please try performing these operations in the command line (multichain-cli) so we can rule out issues with the library. If they still don't work as expected please post a full transcript of each operation performed so we can see exactly what you're doing.