there will always be people who do not get credit for offers. since we're an incentive network, people are able to actually complain about not getting credited (as opposed to people who are just clicking an ad and have no incentive). this makes it seem like it's more of a problem than at other networks, but that's not really the case.
there will always be issues because tracking has inherent flaws. i know of several antivirus programs for instance that have features (some by default) to block tracking pixels for "privacy" reasons. browser settings, ad-blockers, and other things can do the same. there are many reasons why a pixel may not fire.
if you've ever seen an incentive site (like 3604free or similar), you'll see that they have a feature to allow members to request manual credit when the tracking fails. for instance they sign up for an offer that requires a credit card, pay for the offer, and still don't receive credit. they can send in their order confirmation and 3604free will forward that to the advertiser and request it to be credited. even the advertiser won't have the lead recorded on their end, but they'll review the request and credit it if it's valid. we used to deal with these (credit requests) on our network. we don't anymore (because of the hassle) but they're still used by other incent sites & networks.
it's the same way with email submits or anything else, except that obviously a manual credit request is not practical.
long story short, there will always be people who don't get credit, no matter what offer or what network you're working with. it's not an indication of an issue that can be fixed, it's just a given with the industry.