I have an Elvis that was routed in the past - BOTH Left Flippers occasionally or randomly will not work.
I would say the left Flippers work 95% of the time but occasionally pressing the Flipper Button doesn't activate the Flippers.

The Left Flippers have a Single Switch Stack on Elvis - so only one switch closure activates both the lower & upper Left Flipper at the same time.
The Right Flippers have a Double Switch Stack and I haven't had any issues with the right Flippers.
According to the manual this is the way the switches are supposed to be.

As a note - a couple of times I have had it where you press the Left Flipper Button and the flippers don't move but when you release the Flipper Button they will Jump up once and come back down.

I have replaced the Left Switch Stack with a new one from Pinball Life as I though that was the problem abut apparently it isn't.
All the Wires to the flipper coils and EOS switches seem good - no crappy loose solders or anything.

The Left Flippers may not work more often when I am in Multiball - but maybe that's just coincidence.
I would say when the Left flippers work they seem a little weak but that's probably just because they need a rebuild.

In Switch test I haven't had the switch fail to activate so far.

Any thoughts on what to look for?

Yes checked switch and replaced it - it appears to be making good contact.
Wires have solid connection at cabinet switches and EOS.

Any other ideas?

They always register in test when I had it in test, but it’s very random that the left flippers aren’t working and after pressing the button 200x in test I just assume the switch is always closing.
It seems to me that the switch always activates but the flippers don’t always flip if that makes sense.

Also as a note I moved the brown ceramic disc component (not sure what it’s called) from the old switch lugs to the new switch lugs as the new one didn’t come with it. Not sure what the function of that component is.

Both left flippers are in sync so I’ve ruled out mechanical issues based on that.
Both the Top left and bottom left flipper both either work or don’t work together. I’ve never had just the upper or lower not work independently.
So it seems like a problem with getting power or signal but definitely not a mechanical problem.

Also would like to ad this flipper problem has been present since I purchased the machine so previous repairs are unknown - can’t be sure someone didn’t fark something up working on it previous to me.

No other driver board to swap in. Is there 1 or 2 transistors for the 2 left flippers? Do you know which transistor(s) I am looking at?
As far as test I was only testing the dedicated switch. I wasn’t in coil test.
I haven’t checked voltage at the coil - how does one go about that? And what should the voltage be? I will check that.

Shout out to everyone for the help. Cnypinballguy suggested I put a continuity tester on the switch and leave it on during game play. Well I found out that the new switch wasn’t making contact when the flipper wasn’t working - so I used a burnisher on the contacts of the new switch and now all seems well after about 15 games no problems - so hopefully it’s back to 100% now.

So apparently as mentioned above a new switch can also have problems.