The luteal phase (the time between ovulation and menstruation) does not usually vary much from cycle to cycle, while the follicular phase (the time between menstruation and ovulation) can vary considerably. A variance of more than a day or two in your luteal phase usually means that ovulation may have occurred earlier or later than your chart indicates on one or more cycles and you may need to make some adjustments to your charts. You can use the manual override feature or the ovulation detection tuners to make adjustments.
Alternately, if you are unable to determine a more likely date, you may need to exclude one or more cycles from your statistics.
If it is taking a long time to conceive and you are sure that you are charting accurately, a varying luteal phase length is something you may want discuss with your doctor.