Hello!
Logicity is only able to handle this when the main report, and the sub-report share the same database connection. If they are different(and require different credentials) that is a current limitation of the product.
As a workaround, you could use file DNSs(Which can store the password), active directory connection to the database, or configuring the data in a way that it can all be accessed from the same database connection(i.e. using cross database views).
I hope this fixes your problem!
-Devin