Had two Nikko's, sent them both back. Few weeks ago, checked another one in a store, magnification ring didn't stop spinning, you can just keep it turning.
Off course, I've seen really great ones but I generally don't like the idea of changing something until I bump into a good one. I rather change brands.
You can replace it and hope for a better one, but maybe you should get a refund and go for something else.
Where I come from, Nikko is about £100 cheaper than Sightron, and Sightron is a better choice IMHO.
Or, if you are on a budget, Falcon proved to be of a high quality for much less than those two.
I've been using T50 for some time and was very satisfied with it.
But, I'm not a very serious competitor so what works for me, maybe just isn't good enough for you.