Seems newshosting provide 500 days of headers retention, I will be happy if I can just download those.
Except they don't. Their groups contain no more than 120 million headers, which for some groups, is 500 days and for more active groups less. a.b.hdtv isn't even the largest group. So, giganews has 10 times as many headers and 1000 days of retention. That means 120 million is 1/10th the headers of giganews. So, on a good day, I'd guess newhosting has no more than 100 days worth of headers in this group. Considering how volume has gone up, I'm not surprised you can only see 60 days worth. Best case, they have no more than 100 days and your experience suggests real world retention of headers is 60 days.
You understand, with a hard cap of 120 million headers, header retention will be different for each group, the more active the group, the lower the header retention. If they claim 500 days worth of headers in this group, they're lying. I'm not suggesting you change servers, I'm telling you though, you've probably got all the headers you can possibly get with this server for this group. I suggested a cheap way to get the rest of the headers too.
Giganews has a $10 plan with a 50 gig limit but, free headers so, you could download headers from giga, make giga a fill server so, it only fills in parts that are missing on newhosting and then you stop downloading headers from Newshosting but, still download most of your files from them. I'm not really promoting giga, I get nothing if you sign up but, they do have the best header retention of any server. Alternatively, you sign up for a month, download all the headers you want and then cancel, while maintaining the headers you downloaded on your PC and simply topping them off every day.