Adjuvant loading in product formulations are at low concentrations and are fixed to tie to the chemistry they are mixed with. Sometimes there is not enough adjuvant to reach the necessary typical volume to volume ratio of volume sprayed. Adjuvants treat “water” not acres and some pesticides will not donate enough adjuvant to provide needed performance upgrades even if you think they are “loaded”. Another thing to keep in mind is that some generic products have a lower percent of surfactant, so adding additional surfactant may help.
Always read the label and make sure anything you add is not discouraged or prohibited.