There were a number of delays due to weather and instrument issues, including last night.
Now ULA has issued the following statement.
The launch of a United Launch Alliance Delta IV Heavy carrying the NROL-44 mission for the National Reconnaissance Office was scrubbed today when the terminal countdown sequencer rack (TCSR) identified an unexpected condition prior to the engine start sequence.
The TCSR, which controls the final 10 seconds of the countdown, performed as intended and safely initiated a hold at T-minus seven seconds. The team is currently reviewing all data and will determine the path forward.