It is possible that a plant is not always fertile. There can be temperature sensitive mutations that cause sterility. However, the temperatures would be those when the gametes were developing, many weeks or possibly months before the flower blooms, not those of the same day. Temperatures the same day can also have effects. For example too high a temperature can have negative effects on pollen.
'American Freedom' has 29 registered offspring to 2017. Of those it was the pod parent for 10 and the pollen parent for 19. Although that suggests 'American Freedom' might be easier as a pollen parent, a statistical test indicates that the values are not extreme enough to be different from the expected 50:50.
Of the ten registrations as pod parent, three were by Smith in 2006, 2007 and 2008. The other seven were by several hybridizers beginning in 2009, 2011, 2012, 2013, 2014, and 2016 (2). I have noticed previously that when a cultivar is a difficult pod parent it is its hybridizer who registers seedlings using it as a pod parent - I assume because the hybridizer will have the most plants to use as pod parents. In the case of 'American Freedom' that does not appear to be the case.
If 'American Freedom' is quite slow to open its flowers it may be that the pistils/(stigmas) are no longer receptive by the time the flower is open properly. In that case may be pollinating the flowers the night before, the morning before or perhaps even earlier might be more successful?
Have you tried potting it and moving it into the house or under shade for pollinating?
