SuStel:

That's your error. During the "once," they were still
elves. When it was "once," being elves wasn't completed.

hmm.. Perhaps I'm beginning to understand. So you're saying that the
event, which is described by the aspect suffix {-pu'} must :

1. be completed
2. have stopped being in effect

right ?

No, it needs to be completed by the action to which the verb the suffix is attached to refers. If that very verb doesn't describe the completion of the action, then the completion suffix isn't appropriate.

Suppose I was an elf yesterday and I'm an orc today.

wa'Hu' elf jIH
I was an elf yesterday.
During yesterday, I was being an elf. I didn't complete being an elf during yesterday. I completed being an elf after yesterday, but I didn't say DorDI' wa'Hu', I said wa'Hu'. So being an elf was completed, but it wasn't completed during yesterday.

wa'Hu' elf jIHpu'
I was (and completed being) an elf yesterday.
During yesterday, I was and elf and then my being an elf was complete. I stopped being an elf during yesterday, though I had been one before that.

-- 
SuStel
http://trimboli.name