Hmmm.... it doesn't matter whether 60% or 99% of cyclists think "on your left" means "let me by."
If the first callout didn't work, and especially if the second one didn't, then I wouild assume that for whatever reason, "on your left" didn't mean "kindly let me by" to them... and I would be more explicit in my instructions.
Depending on my mindset, my tone might indicate "obviously you are an imbecile because I have to tell you this" ("Excuse me, if you will go single file for a few brief moments, then you will not be blocking the road from faster riders") or more casual & friendly ("hey, could you let me around?"... tho' perhaps iwth "that's what 'on yer left" means, ya know")
It's *really* easy for people who usually aren't jerks to temporarily reside there. I do it myself.
(But... the first time somebody said "Bikes back" on me, I'd remember "oh, how rude I was!")