There might be anywhere an error which breaks the transfer but this error didn't reach Qlik and therefore Qlik is further waiting to get any result (forever respectively until any timeout is touched).
If you could ensure that the query is executed and send from the database it means that the driver and/or the network caused this issue. You may run into any network/driver-timeout or the driver couldn't handle any data.
Therefore I suggest to check the run-times and if there are any customizing options by the driver/database.