The coverline- the horizontal line drawn across your chart after ovulation has been detected- carries no physiological meaning. It is just a visual tool to help you see your ovulation pattern. Whether or not your temperature rises or falls below this line does not necessarily indicate anything significant. Individual temperatures can fluctuate at any time of the cycle, for both hormonal or non-hormonal reasons. A single dropped temperature usually doesn't mean much. You want to look at "the big picture" and see a pattern of temperatures over time. As long as you can see a biphasic pattern and a trend towards elevated temperatures after ovulation, there is no need to worry about a single dropped temperature or two.
If, however, you have so many lower temperatures that you can no longer see a biphasic pattern on your chart, (and you are taking your temperature accurately) this could mean that you did not yet ovulate and should consider yourself potentially fertile so you don't miss an opportunity to conceive.