The Elder Wand not working for Voldemort? Say what?

[Carol]

No: HPFGUIDX 181709

What I want to know is how Voldemort "knew" that the Elder Wand wasn't
working for him and that, therefore, he wasn't its master. the wand
seems to me to be working just fine. It certainly killed a lot of
people (not to mention the soul bit in Harry) and it created that
floating cage around Nagini. Where do we have any indication that the
wand has failed him? (snape says that Voldemort has done
"extraordinary things" with the wand, though what they were and how he
knows that, I have no idea, and Voldemort explains that its his own
extraordinary abilities, not the wand's that made those "extraordinary
things" possible?

Huh? None of it makes any sense. If, indeed, he accomplished
extraordinary things with the Elder Wand, which seems to work fro him
just as well as his own wand ever did (and Ollivander says that the
yew wand did "great but terrible" things--including making Horcruxes
that Ollivander doesn't know about), what is Voldemort's problem? What
flaw does he seee in the Elder Wand? It doesn't stop working for him
until after Harry's self-sacrifice and return from his visit, in
spirit, to King's Cross. (And, yes, I believe that his spirit was
really there and could really have "gone on," leaving the Voldie soul
under the bench, had he so chosen.)

Had Voldemort not noticed that the wand wasn't working, he wouldn't
have summoned Snape. And yet, as far as I can see, the wand is working
just fine, and there's no need to summon Snape at all (which also
raises the question of how Snape was supposed to find out about Nagini
in her bubble so that he could pass his message on to Harry).

Carol, annoyed by this flaw in the storyline because it makes no sense
at all