During configuration, if the DOUT pin never goes High, this might indicate that the configuration memory is not full.
In order for the FPGA to enter the Startup Sequence and complete the configuration process all Data Frames must be correctly loaded into memory resulting in the memory being full. DOUT goes High to indicate when this has occurred.
A possibility might be that the configuration process was not allowed to complete or was terminated too early, or perhaps no configuration data was even sent. Check the number of configuration data bytes that have been loaded.