What do you mean? Did you earn 210 nights last year without rollover from the previous year, or were these points that had rolled over from a previous year? If it's the latter, you didn't lose anything. That's how it works.
Nights only rollover once. Only nights earned in the calendar year above 75 rollover into the next year. If you rollover 100 nights and earn 60 nights the next year, the year after, you will have no rollover nights. Despite having 165 nights showing for the second year, only 65 would count towards rollover, which does not reach the required 75 night earned-in-the-year threshold.
So if this is the way it works, what are rollover nights good for? I went from 600+ in December and only have 279 now. If I don't stay more than 75 nights in 2015, I guess that means I'll have zero next year? I don't understand the point of rollover nights at all then. Those nights don't do anything for me - do they?