Some things to consider...
Do you always use the same browser for signing up, and later confirming the email address, on a given offer? If you sign up in IE and confirm in Firefox, you wont get credited (IE and Firefox cookies are stored separately). Also note that you can only confirm an account once -- if you mess this up and then try to re-confirm in the correct browser, it will not credit you (since you already confirmed your account).
On the same note, if you clear your cookies AFTER signing up but BEFORE confirming your email address, it will almost certainly prevent you from getting credited.
If you use an online service to disguise your telephone number, or if you use a "throw-away" voicemail box, advertisers may reject your signup (silently).
If you use a program like RoboForm, advertisers may reject your signup (silently). It makes you look like an automated computer program trying to scam them! And yes, of course there are ways to detect it.
Funny tricks with your email address, particularly using periods (.) or the plus sign (+) in your Gmail account may also cause you to not be credited.
Sometimes an advertiser has many different websites with different names. Without you knowing, you may be signing up with the same advertiser for the 2nd or 3rd time, although on a different website. When this happens, some advertisers will silently prevent you from being credited, as they consider the registration a duplicate. When possible, FusionCash automatically hides offers which exhibit this behavior (that is, if an advertiser owns offer A and offer B, and you complete offer A, FusionCash automatically hides offer B from your view). The limitation to this functionality is that we don't know what offers you've signed up for on other sites, or through Google, or a blog... you get the picture. We always try to do the best we can with the information available to us.
If you visit an offer via Google, or a text ad, or a banner, or another rewards site, or any other way: you run the risk of not being credited should you later decide to signup on FusionCash. You must clear your cookies BEFORE signing in to FusionCash to prevent this. I do not recommend clearing your cookies once you have signed in. Cookies from different offers are designed to coexist peacefully (and they do). The technical rules regarding cookies prevent one website's cookie from affecting, in any way, another website's cookie.
I hope this provides some insight. Bottom line: clear your cookies before you sign in, then complete one offer at a time, filling out forms by hand with completely valid information. Double-check your answers and be absolutely SURE that you are visiting confirmation links in the same browser that you used to sign-up.