Ballons have a low RCS(RADAR CROSS SECTION), typically NORAD uses the over the horizon early warning radar. These radars are in VHF, UHF which can see the balloon but the software in them was designed to ignore slow moving radar returns. Otherwise every bird flying will be on the radar screen along with noise clutter.
Further more these radars don't give accurate BRA (BEARING RANGE ALTITUDE) since they work in the lower frequency band they have extremely good range but poor resolution. This is the reason why US didnt want to give F35 to Turkey because their S400 battery as a VHF radar, but tailor made to see targets with low RCS.
But to destroy the target, you need a tracking radar like the one in Aegis equipped ships or PAC 3 or THAAD. These again have the same problem the software is not designed to track slow moving targets with really low RCS, even though they have some really high powered X band radar which can spot the ballon from several thousand miles away. May be the software needs to be tweaked or some body manning it, needs to be retrained. Since there is no bird flying that high and it has to be a man made object.
In saudi arabia, the refinery was targeted by Iranian drones which are slow moving and the US patriot sites couldn't stop them. So there is certainly some limitations.
The public will never know.
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