My first guess here is pH problems. Off levels of pH will block certain groups of nutrients from being absorbed.
Your plant may also be suffering from overwatering or heat stress. As the grower, you will know by your habits which of these is probably most likely. If you have a fan blowing directly on them, that could also be the issue.
Final thought: plants will drop 1-2 hours before lights off. If you're on a 24/0 schedule, you may see your plant dropping for 8 or more hours a day and be confused. In this case, don't worry, it's a natural circadian rhythm.
You're not in bad shape, your girl looks happy and mostly healthy, keep up the good works!