This link by Military Times gives more details on the slow roll out of the National Guard troops. It took a number of hours from getting top level Administration/Military sign-off, set up at the DC Armory, and then movement to Capitol Hill in time for the 6 pm curfew the Mayor had decreed. What time did the Republicans at the Capitol call President Trump and ask for aid as rioters were screaming “hang Pence”, had already broken windows to get in, etc.