So we are made to believe that the villain searched the attic every night for months in a row and never found or examined the dress??
Very good film but this, for me, is rather a big plot hole.
Or did I miss something?
No, you didn't really miss anything.
I generally define a "plot hole" as something that can't be explained reasonably. Although this is something that could be called unlikely, it's not unexplainable.
That costume was an item with which he'd been familiar long before he ever killed Alice Alquist 10 years earlier - he'd seen her wearing it while playing the empress ("Her greatest role," said Paula); she's wearing it in the portrait - so it had simply become something that was so familiar to him, he overlooked the obvious, hiding in plain sight, until the light hit it just right.
Not unlike when you've misplaced something around the house, and when you finally spot it, you realize you've walked right past the very thing you're looking for a dozen times. And when you do, it may seem unlikely ("How could I have missed that?"), but it's nevertheless common.
And without the unlikely, most of the drama we know wouldn't exist.
Poe! You are...avenged!
I had similar thoughts. Not so much about the dress alone, but that he was searching through the drawers and such as if it was his first night in the attic. It would seem after a week at most he would have had searched through pretty much everything up there. Figure he was up there 6 nights a week, about 2 hours each time, for at least 4 months. That means by the end scene he had spent around 200 hours searching the attic!
"Who's running this airline?!"
Until it got to that point in the movie I thought Gregory was going up to the attic as part of his attempt to drive Paula crazy. He wasn't making the noises and messing with the light for those reasons; he was just searching for the jewels. Why wouldn't he be concerned the noises would wake her? Her room is directly under the attic.
One of the problems I had with the movie is how easy it was for Gregory to convince Paula she was losing her mind. Yes, she had been through a traumatic event in her youth. That might make a person 'fragile', traumatized, and unable to cope but that did not seem to be the case with Paula. At the beginning of the movie, her music teacher, Guardi, tells her she's too happy to sing a tragic opera.
She is supposedly in love at this time but it did not come to him immediately that she had suffered a tragedy of her own. This leads me to believe even before her new love Gregory came along, she was doing pretty well; not unhappy and traumatized. I know those corsets are tight and women in those days were prone to lightheadedness but to convince someone so thoroughly that they're losing it - and over the most minor things when they had been perfectly fine prior to that, is hard to believe.
When Paula begins to notice the dimming of the lights, why wouldn't she put that together with the noises coming from overhead? At one point she says she checked the entire house but clearly not the attack. If she had, she'd have noticed the rummaged condition of the items Gregory had been through.
Why wouldn't she realize all of her lapses involved Gregory or were reported to her by him. She had figured out the lights dimmed after he left and always came back just before he returned. It wouldn't be hard to link all the odd occurrences to him.
She seemed to flip out over the smallest things but when she was finally in the presence of many other people why would she blow it? She had finally stood up to Gregory and was so determined to go out and be with other people she was going to go it alone. One would think she'd do everything in her power to stay there that night.
There are a lot of unexplained things in the movie but the fact it took Gregory so long to find the jewels was not an issue for me. There were a lot of items in the attic; a whole house full. He was looking for the jewels or anything that would indicate where they might be hidden. That could take time and since he hadn't yet found them, he'd continue to search. They were always in plain sight but it took the exact right conditions for him to notice. That night it was the moonlight hitting one of the jewels in just the right way.