Regardless of the amounting and planning, calculating, test runs and theory - sometimes actual practice just doesn't pan out. When the San Diego Trolley first started operations they had planned on 15 minute headways using automatic switches at sidings and limited double track. They only ran the schedule about a week before reverting to 20 minute headways. Once phase 2, double tracking the entire initial line from Union Station to San Ysidro was completed, they quickly established the expanded schedule.
Another issue may be enough qualified personnel. SMART has been advertising/listing/recruiting a similar ad now for months and doesn't seem to be letting up. You would think they should have found plenty of qualified people by now especially considering the wage being advertised - or are they being too picky of the applicants or isn't it enough to live in the region? SMART ad link