I had the same problem.... I couldn't get my nitrates below 80 so I did a lot of Googling to find a solution. A BRS video said to do a 30% water change, then three days later do another 30% water change, and then three days later do one more 30% water change. I did that but the nitrates went right back up. We cut back to feeding every other day (which the fish are not happy about). A BRS investigates video said to replace the filter socks every 3 or 4 days, which I do, because after four days the nitrates start going up in their experiments. (I sew my own so it's a cheap fix). And I have a giant ball of chaeto.
But nothing was really helping.
Then last January I came across a Dr. Tim video on nitrates from MACNA last year and in it he said that your skimmer kills your good bacteria.
So I immediately turned the skimmer off. I did the three 30% water changes again and my nitrates haven't gone above 20 since. I can live with that.
I still try to replace the socks every 3 to 4 days, and I'm lucky if I have time to do a 20 gallon water change every week, but so far it's working fine for me.
I've been wondering if I should just pull the skimmer out since I haven't used it in months and add more live rock in it's place, or put the skimmer on a timer so it only runs for, say, two hours after we feed the fish. I guess that's worth an experiment to see if it helps.
Hope this helps!