Question: Where are the affiliate sign ups coming from? What are they being told before they get to your sign up page?
Once you pin point that, then you can get into their heads and figure out their hopes and desires. It may not necessarily be a matter of motivation, but perhaps a matter of qualifying your affiliate sign ups more closely.
Affiliate sign ups are the same as qualified leads for prospective buyers. Both bring in money and you don't want just anyone.
You want to make sure that your affiliates are a good match with you!
Hope this helps!
All the best
Enigma Valdez