Antivirus missing threats is completely normal. There is no defence that can provide 100% protection neither from stealers, nor from anything else. Detection should always be balanced with performance and lack of false positives and this balance is what criminals exploit to evade detection.
For example, the more you optimise the antivirus for lightness, the less emulation your engine performs (there is no other way) and the less API calls the behavioural blocking monitors and reports. These are just natural limitations not only of K7, but of the whole industry. But or course, some miss more than others — K7 is more on the miss side than let’s say Kaspersky, Bitdefender or Norton.