5E3 Speaker Wire Weirdness

Howdy! I have a Mojotone 5E3 I built back in July. Never had any issues, fired up a-ok first try. Like all of my amps, I use it with a Weber Mass attenuator, and have successfully done so since building it.

Today I hooked it up to the attenuator with a different speaker cable than the one I was using last week and got no sound, nothing. Switched back to the last cable I'd been using (between the main amp out and the attenuator), and it works just fine. Long story short, about half of my speaker cables don't work when placed between the 5E3 out and the attenuator, but they all work with my 8 other amps when used for the same purpose. The two that do work are slightly longer (5' and 6') than the ones that don't (2' to 4'). In addition to plugging them all in, I've also verified the speaker-to-attenuator connection, made sure it works plugging the speaker directly into the amp out, and wiggled/twisted the non-working cables around when connected to no avail.

Any ideas on what's going on--maybe something super obvious I've missed, or something about the 5E3 design that would make this the case? This seems really weird.

 

Everything's clean and the attenuator and cables all work fine with all of my other amps.

Even more weirdly, I took the cables that don't work with the 5E3 and plugged them into a 1' extension speaker cable (1/4" plug on one end, 1/4" female jack one one end) and that combination works with the 5E3. This is bizarre--could it be some kind of impedance/load thing?

 

Thanks everybody, it turns out you're all correct! The main speaker jack is the switched kind that has a third terminal with a plate that's tensioned to touch the tip "arm" when no plug is inserted. The 4 speaker cables that weren't working seem to have plugs that weren't big/long enough or shaped right to separate the 2 plates when the plug was inserted.

BUT now I'm confused on what my DMM should be reading on these connections. I would think there shouldn't be continuity between the tip and sleeve of the speaker cable once it's been inserted into the jack but there is continuity when inserted, which is the same with all of my amps. And there's continuity between all 3 lugs of the switched speaker jack with no jack inserted, even when the switch and tip are held apart. The thing is working, but now I'm confused!

I'm trying to remember the purpose of having this jack switched. Is it to ensure that it's grounded if no speaker load is connected, or does it have to do with the 2nd speaker jack?