YPN now has more publishers than a year ago, meaning advertiser dollars are going to run out quicker, thus lower paying ads are showing on your site earlier in the month.
Chances are, that is what is happening - your sector is getting diluted by other publishers/websites who thought, hey, that's a nice money making idea... I'll do that!
Other 'problems' might consist of:
- Advertisers bidding less
- Major advertisers which were there last year simply jumped ship
- Your impressions aren't 'quality' in the eyes of YPN thus aren't showing the higher quality ads
- Are you geotargeting? If not, you should definitely invest your time into implementing some sort of geotargeting system. YPN only pays for clicks made from within the USA.
You also said your impressions climbed 10x, but the clicks only climbed 5x. Why is this? Are you targeting something different, your visitors coming from somewhere else, etc? Usually that isn't a good sign. The impressions and CTR should climb together, if they aren't - there is usually a problem with either the type of visitors your site is receiving or possibly the new keywords you are targeting. As well, the geotargeting might help in this instance. Are you showing YPN ads to all your visitors, or just the ones inside the USA? That could be apart of the problem if you are showing to everyone.
Have you tried other publisher networks? It seems that YPN is your only program you are in from what you have discussed. Try out Adsense, Adbrite or one of the other networks.