Since this thread is in the "New Hobbyist" forum:
I think sometimes experienced hobbyist bring up these types of issues starting in the middle of the conversation instead of the beginning. Most of us understand where you are coming from, but those new to the hobby may not and may cause their thinking to lack fundamental understanding.
The responses I read here do not address the question of "common causes of persistently high nitrates".
Faulty or inadequate equipment, lack of nitrate consumers, etc do not cause nitrates. The responses here describe how we deal with nitrates, not any causes of nitrates, which was the question.
There are several method we use to either skim off organics before they convert into nitrates and/or export nitrates once they occur.
The cause of persistent nitrates is persistent life. Nitrate is an end product of metabolism. Deal with it.
Yeah, I'm being picky, perhaps the question should be reworded to something like "What are nitrate reduction/export best practices?"
-I'm working out my last week of my notice here, starting my new job next week....bored to death, otherwise I wouldn't have enough time on my hands to be so picky...