The first scheduled launch attempt was for 27 January 1964, with all going well until 93% of the liquid oxygen (LOX) had been loaded into the first stage. At this time, the ground crews switched it from a fast fill to a replenish system. However, the LOX in the tanks began to fall, i.e., it was not being replenished. The cause was found to be a blind flange (a plate without an opening) in the line. This could not be removed easily and the launch had to be postponed for two days.