As a traveller to the US (in the past but maybe to Hawaii in the future as part of a travel bubble), the (supposedly dicrestionary!) tipping system is quite hard to comprehend and get it right. Asking them what is the normal amount gets blank faces!
I'm surprised you got blank faces. It's 15% though some places are trying to raise that to 18% or even 20%. And yeah, it's "discretionary" in the sense that you CAN refuse but you will be hated on for it. [some restaurants will include it in the bill, however, especially if it's a large party. Watch for that, if the tip is included you have no further obligation.] Some advice I got once: if the service is just
awful rather than leave
no tip (which will be blamed on "gee aren't Aussies assholes" [no, you're not]), leave a very SMALL tip. That sends the message, unambiguously, that they suck--you knew you were supposed to tip, and yet this is your assessment of their worth. However, I'd never do so to a waitress/waiter if the fault clearly lies with the cook. I'd complain about the cook some other way.
I personally tip well over 15%, especially on a small check, or if they seemed to be busting their tails. Back in the day you could get a very cheap $4 lunch at some restaurants (that would make it up for dinner which would be $12 or so), I'd tip as though paying "normal" price not the cut rate lunch price; I'd tip a couple of dollars.
I've been "bitten" by not understanding customary tipping in other places. I fear on my last overseas trip I pissed off a lot of the locals because I was under the impression there was no tipping. And even when someone suggested I should tip so-and-so, I fear that I tipped too little. I have no idea. It was "third world," though, so the money I tipped might have been HUGE to them, for all I know.