Certain big site seems to be cheating?

[pos777]

A certain web cam site I affiliate with is lowering the signups I get now that I’ve changed to Per-Sign-Up. I was getting 5 to 8 sign ups a week on the % program, but this dramatically dropped to 2-3 per week when I changed the code to Per-Sign-Up. (I did this to get some ready cash to pay a hosting bill.) The traffic level has remained constant, nothing has changed apart from the program. The same thing happened last month so I changed it straight back to %.
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Has anyone noticed this? What can I do? Do I approach them? I’m still quite a newbie…