Outside of balloon launches being detected by other sources and methods, what should have detected it would be the Cobra Dane 100' diameter planar phased array radar at the end of the Aleutian Islands - assuming there's enough metallic matter in the craft to create reflections.
The most recent balloon with its 100+ foot boom, solar panels, antennas, electronics, batteries, etc likely had enough metal to create a good reflection, and thus detection (assuming relatively low velocity events are not filtered out). That radar, along with others that are similar, were designed to detect ICBM launches from the USSR/Russia (both missile development tests re-entering near Kamchatka, and of course the real thing targeting the US mainland).
But...if the previous balloons did not have the long boom and payload (and thus little reflective metal), and were just trial runs to test detection responses from the US, that *could* be why they were missed. Just some conjecture....