There can be several reasons for this and it does not have to mean that you did not ovulate.
femSense works most reliably when your cycles are regular. However, cycles can sometimes be shorter or longer. In some cycles the follicular phase (the first half of the cycle) is a little longer or shorter than usual, which means that ovulation also occurs a little later or earlier. This is completely natural, but it can still affect the accuracy of femSense.
If the patch is applied too late or too early, ovulation may also not be confirmed.
Another cause for this may be an anovulatory cycle.