Sponsors - which affiliates really check what works?

[cots]

You know I run Glam Images/Ult Fetishes and, as much as possible, I am a stats freak. In my daily/weekly trawl to see what's new, I am always intrigued that some TGP/Directories constantly feature sponsors that don't sell for me.

In fact, some featured sponsors have never generated just one sale at my sites. Others have been in major decline over the past few years to the point that I hardly feature them any more. Quite a few of the site owners (sponsors and affiliates) are here - so I won't name names.

Questions. Assuming my content matches incoming surfers expectations -

Do sales (based on conversion not volume) widely differ from TGP to TGP?
Do some TGP owners just go for signup volume and no other criteria?
Do some webmasters, despite protesting how bad business is, just not care enough to look closely at the reasons why?

Am I missing something obvious?

The main reason I ask is that over the past three years or so GI/UF revenues are down 40% (but still very, very reasonable). Traffic is up 20% in the same period. Most of the decline in sales comes from -

Closed sites - nothing I can do about that
Not enough new sites - Good new sites still have fantastic sales
Good active sites that have a lousy affiliate content update rate

But the real killer are sponsors whose conversion has just really slipped. If I look at my Top 10 from 2009, and compare them with 2012, there have only been 2/3 sites that have kept up. Others have just dropped and dropped and dropped. Good clicks still but lousy conversion.

Lets hear from both sides here (after all I am a sponsor too!).

Confused from Tenerife