If points are deducted for not having turned on or off some lights, such as your taxi light, this page might be helpful to you.
First of all, check to make sure you do turn on/off the lights at the correct lights at the correct time. For more information about our flight scoring, please see the complete guide to flight scoring.
If you are sure you have followed the instructions correctly, there might be an addon detection issue. Unfortunately, not all addons report their lights' status to FSUIPC correctly and as such our flight processor will incorrectly believe lights are turned off while they are actually turned on and vice-versa. Some add-ons, such as the PMDG aircraft provide additional FSUIPC data points that allow us to identify the correct status of these lights. However, in most cases these data points are not available and we have thus far not been able to find a proper solution for this issue.
For some aircraft, such as the TFDi Design 717, FSLabs A320 and the FlightFactor A320, we have disabled taxi light points and as such you will always receive full points for the taxi and strobe lights on these aircraft. There are additional aircraft that also have these fixes, such as the QualityWings 787 for the landing light. The list given here is not complete. If you believe your aircraft addon to be malfunctioning, please create a support ticket so we can look into adding your addon to our ignore list.
Alternatively, or if your aircraft addon is a supported addon but does not function for you specifically, one of our pilots (Jose Serro) has kindly provided us with a program to partially solve this issue. By using his program you can manually force the FSUIPC data points/offsets to a specified state. For most addons, this should resolve the issue if you set all lights at the correct positions at all times. It can be downloaded here. Instructions to install and run are included in the ZIP file.
For PMDG aircraft, please see the Prepar3D aircraft support guide.
For the FlyByWire A320NX, please see the MSFS aircraft support guide.