What type of database is it that you are connecting to?
Are you using the Data Manager or the Load Script? The Data Manager has some nice features, such as data profiling and auto-suggestion of joins, but it is not always as efficient.
If you are not yet trying it I would definitely opt for the Load Script.
Database loads can be affected by record locking, in exactly the same way as loading rows straight against the database, so that is worth checking. The fact that two separate tables suffer the same fate this sounds less likely.
Connectivity between Sense and the Database could be an issue. Is there any port-blocking between the two - it could be that the authentication is done on one port and the retrieve on another.
Hope that gives you something to try.
It looks like this is an issue with the iseries access ODBC driver -- VERY SLOW script to load from ISERIES ACCESS ODBC DRIVER in QlikSense April 2018
Sounds like we will just have to wait for a fix to be released.
We have not had any success at this point. We know that our server setup isn't ideal and will make changes down the road, but we will need a BI solution that can work with our existing tools as we transition.
For now we are using Excel files instead of a live database connection to allow our evaluation team to get a sense of what they can expect if the data connection issues are resolved. Hopefully a fix will be rolled out soon enough for us to assess Qlik's viability from an IT perspective.