And I'm livid... to have waited on the phone for 15 minutes to be told that "we can't find your bag, so gather up your receipts for your items in the bag and we'll reimburse you."
How about "NO." Just find and send me my bag, it's not that hard.

Story is, I flown overseas from Asia on Cathay Pacific airlines to LA airport, and had a connecting flight to florida. When I landed at LA to claim my bags to recheck them, one of my bag was gone... after waiting for over an hour. I spoke to airline representative by the Cathay Pacific airline carousel at the time, and he reinsured that he'll meet me in an hour to let me know the updates... he never showed up.
I flew back home in hopes that someone might have picked my bag up and rechecked it instead of taking it home, but I ended the night in disappointment and at the JetBlue baggage claim office. The JetBlue representative told me that I'll get a $30 voucher for my inconveniences, and it'll be delivered to my house when found.
Fast forward a week and a half later... No sign of my bag. I just called JetBlue and the incompetent manager of my claim case was unable to coherently tell me any information about my bag, other than "we'll still look for it."

I'm at my wits end and without my valuables for almost two weeks now and I'm not sure what happened to it.
Only explanation is that someone must have taken my large suitcase...
Or it's still sitting in LA airport somewhere.

But I hope it's the latter, or just got transferred to another destination.
Should I also call and contact Cathay Pacific or just leave it to JetBlue? I was told that the last airline is responsible for lost/damaged bags. But it's been so long, even if found, my bag must be in the dump by now.

EDIT: Case in point that she was "incompetent", calling me back 5 whole hours later, claiming she got my email wrong. I then had to stay on the phone with her (I requested so) to MAKE SURE she was able to send an email right this time... Oh, it only took 6 minutes for her to send the same email. No biggie, she just suck at her job.

Bags disappear. Some times they get stolen. That's why the joke goes: There are two kinds of luggage: Carryon and Lost.

My bet is it's gone. But JetBlue will still look for it. Not sure what the manager of your claim case would have to do to become "competent" in your mind, find the guy who stole it (or took it by accident)?

And frankly, if it wasn't there in LA when you got there on Cathay Pacific, and you were responsible for getting your bags and rechecking them, how the heck is this even a Jet Blue issue? OF course you should be contacting Cathay Pacific!

If the bags were tagged through then, technically, it is JetBlue's responsibility as the final carrier in the itinerary. But I would agree that pressing Cathay Pacific is a good idea.

She was unable to tell me any information regarding my baggage. She was barely articulate and most likely unable to process any information I was telling her regarding my experience.

Like I've said, I spoke to a Cathay Pacific representative when I've realized that my bag isn't coming. I also questioned whether or not I should be speaking to Cathay instead when I was on the phone with JetBlue, what I was told (and have already mentioned) it is airline policy that the last airline I've flown with, is the airline that is responsible for the last baggage. It is apparently no longer Cathay Pacific's problem.

You're right, I should contact Cathay Pacific as well to see if I can get any help and resolve this situation.

OP said when they landed in LA, they went to claim their bag but it wasn't there. I don't see how this has anything to do with JetBlue.

Because that's the way IATA rules about checked bags work. If the bag tag had the B6 segment on it then B6 is the final delivering carrier and it would be on B6 to work with the prior delivering carrier to locate and eventually deliver the bag.

And if the bag was stolen, which I'm guessing is the case. Cathay and Blue could work their butts off and still the OP is going to be upset.

Then why did OP state that they went to claim their bags to recheck them, if they would have been transferred automatically?

post a message on THEIR fb and/or twitter and/or instagram!?

Clear Customs?

Because US customs requires all passengers to accompany their bags* when entering the country. That doesn't change how the bag is tagged at the original check-in counter.

*With very few, limited exceptions which are not relevant here.

I had pre clearance in my mind for some reason. My fault.