I mean, after killing Snape, shouldn't Voldemort have found that the wand in his hand still refused to show greater power for him? Try several random spells, the Dark Lord would find the wand works no better than when Snape is still alive, then he should realise there is something went wrong...
It doesn't seem like he did much after that.
All he seemed to do was magnify his voice, cast the killing and Cruciatus curses, and a couple of silencing charms, none of which can visibly be lacking in power.
He did fight in the Battle of Hogwarts, but like PMar said, he wouldn't have noticed.
He frequently was too confident in his own theories and ideas.
He's acted (to his downfall) like this before; leaving the diadem in a room that obviously had been previously explored , just because he didn't want to admit he wasn't invulnerable. Proud people often believe what they want to believe.
''Killing'' Harry might've erased any doubts.
When he accomplished something he frequently blamed his wands for not being able to do, he might've felt it really did work well for him.
Voldemort couldn't have noticed that he still didn't command the Elder Wand, because he didn't actually use the wand enough after killing Snape. His first spell afterwards was the one to send his voice to everyone in the castle - and it would be very difficult for him to detect any weakness in that spell as he can see only one side of its effects.
After that spell, he is waiting for Harry in the Dark Forest, so the next spell he casts is the Avada Kadavra he used on Harry. He didn't notice any weakness in his command then because (a) The spell also knocked him out (for reasons unrelated to his command of the wand); (b) Narcissa Malfoy lied and said Harry was dead.
The next time he uses the wand is during the Battle of Hogwarts, at which time he is too caught up in the battle to be able to notice the wand not really working for him. He probably didn't notice until Harry himself pointed it out to him at the end [book, not movie].
I think the premise is wrong: First, the wand was working fine for him; second, he had no other theories as to possible owners; third, he had no idea what to expect from the wand anyway.
Other answers listed various spells he performed and any failures in those spells were not because of the wand, but because of Harry's sacrifice. So up until the moment of Voldemort's duel with Harry, the wand was working fine.
Part of the confusion here is that no one really knows what is supposed to be special about the wand, or is even sure that this wand even really is the elder wand of legend. Supposedly, the brother asked Death for a wand that was more powerful than any other, that made it's master unbeatable in a duel. Yet we know that many wizards have indeed died while wielding the wand. So clearly, it didn't work. But at the moment, Voldemort wasn't in a position to do a scientific comparison of relative power across multiple wands.