Winnie's response regarding the Trip-Free abilities of commonly used MCCBs (Molded Case Circuit Breakers) is why these types of things occur often, with older and newer Breakers.
They have tripped as needed, due to an Over Load / Over Current scenario, only the Reset Handle was not thrown to an "Offset" position (Trip Position being either mid-way "Trip" position, slightly behind the "On" position, or to full Off position).
The tripped device will throw to Off very much easier than the Reset ones (devices in the "On" position being the reset ones).
There is another scenario where an overcurrent does not result in a visibly tripped device - and this in fact will not be tripped at all!
It's when the time-current trip charasteristics of the upstream device causes it to trip before the downstream one does (AKA: Non-Selective Coordination).
These scenarios are really fun (invert the term "Fun").
20 amp 3 Pole 240 VAC 10KAIC MCCB has ground fault - this device sees 2400 Amps for the fault level.
The panel is fed via a 100 amp 3 pole 240 VAC 10KAIC MCCB, which has exact same trip characteristics, and in fact at the +2000 Amp level, the two devices' current lines have already overlapped.
9 out of 10 times the 100/3 will trip only.
Last scenario involves exceeding the AIC of a device - barbequed breaker / welded closed contacting points - no way to trip this device (except to blow it up from a runaway fault or an arc)