the temperature of the place where one is training (a warmer temperature is more conducive to increased flexibility)
the time of day (most people are more flexible in the afternoon than in the morning, peaking from about 2:30pm-4pm)
the stage in the recovery process of a joint (or muscle) after injury (injured joints and muscles will usually offer a lesser degree of flexibility than healthy ones)
age (pre-adolescents are generally more flexible than adults)
gender (females are generally more flexible than males)
one's ability to perform a particular exercise (practice makes perfect)
one's commitment to achieving flexibility
the restrictions of any clothing or equipment