The rise and fall of revenue from domains at Domain Sponsor can be perplexing. First of all, it takes up to 3 or 4 weeks for your domains to "optimize". Optimize means that the links on your page for a particular domain are relevant and return max revenue. For example, I just put GoVenezuela.com on my DS account. So, I would expect, in a week to 3 weeks, to find links to travel related sites. Perhaps some of the links would be specific to Venezueala. Those links should be much better than a random search page: "Our Most Popular Categories", or whatever.
Nothing is static, however. Advertisers come, and advertisers go. So, if you had a high-paying advertiser linked from a domain and then they decide that they have exceeded their advertising budget for the month, or they want to try a new, lower cost search service, or whatever, then your golden domain can stop laying golden eggs. There are many factors that can affect your conversion ratios and your payout per click.
One thing you can do is check your pages for your domains to see if the links that are up are relevant.
Although my average at DS is around $10/cpm (especially if the traffic is US traffic, as most foreign traffic does not convert so well, especially Asian traffic), I have some names that pay out $20 to $30. However, on an inidividual day, an individual domain or even my whole portfolio can be weak. I have seen days when the links were sluggish so that many websurfers might not hang around. In the past, if DS was having a bad server day, they had offered credits to domain owners. However, I have not seen that the last couple of months.