Cases (and gender).
"nasze" is either Nominative or Accusative, either neuter or not masculine-personal plural.
"naszego" is either Genitive (masculine or neuter) or Accusative (masculine animate).
So, as "pamiętać" takes Accusative: "My pamiętamy naszego wujka" (masculine animate), but "My pamiętamy nasze dziecko" (neuter) and "My pamiętamy nasze córki" (not masculine-personal plural).
"We are remembering our uncle" is flagged up as incorrect, but i think that pamiętać is a present continuous verb. I perhaps tend to be overly-specific in my English answers but I find these a helpful parallel, from a learning point of view, when thinking of the polish perfective and imperfective equivalents.