Well personally I don't like to see above 20 since I have some sensitive fish. But I do keep it around 10 for my plants.
And in any tank I would not want to see over 40. Your nitrates should never get that high if you have enough plants.
Edit: OS and I posting at the same time
Are you saying they were stable before and now going up? You said nothing changed but didn't mention the state that you are comparing your new results to. What's the problem you are experiencing?