Odds are that the fort worked rather well in the short term, and that Aquae Granni / Aachen fell for logistic reasons.

Context: the Roman Empire was a mess back then, being split twice (by Diocletan then Constantine), between the fort being built and the city falling for tribals. Couple that with Aquae Granni being mostly a vacation town in the middle of nowhere, and why it fell becomes clear - “too low priority, we aren’t sending supplies there, let the citizens fend off themselves against the bar-bar-bar”.