We need to be very careful before pointing our fingers at different affiliate programs and consider these points.
1) Some players see a banner on your site, and instead of clicking in your affiliate link type in the domain. I have seen and heard of this happen.
2) Players will say they signed up with you even though they signed up with another site just to get the money. As mentioned before they will go out of the way to get that money.
3) Players visit many sites and then signup an account. The question is what affiliate gets the credit. The first one? last one who sent the traffic? You need to ask the programs you are with how the cookie tracking works.
4) Affiliate managers can change the tracking if another affiliate calls and gives evidence that they referred someone who is not tracked.
5) Some players call rather than signup online and this results in tracking errors.
Some programs do screw affiliates, but for the larger programs it will be a pointless move and too much effort so they generally would not.
Since my main income is derived from affiliate commissions I would like to know if I am getting screwed but I also understand the other side of the spectrum.