Where are you looking for pollen on the male blossoms? The pollen is on the center stem in the center of the blossom. It is a sticky like dust.
The blossoms are viable only for a short time in the morning...maybe 3 hours.

Also, if you're getting to them at all after they've opened, it's possible something's eating it or taking it away when flying off. I know you don't think there are any buzzing insects, but there probably are. I find that if I get to mine any later than around 10am, all the pollen has disappeared from the males.

I too haven't been finding any pollen on the anthers, just a little bit of it at the base of the anther. Maybe what's been left over by some visiting pollinator.

I hand pollinate when I can just to make sure. Luckily, I've only lost one fruit so far due to not being pollinated. I just picked my first zucchini today and have half a dozen more growing on 3 of my 6 plants.

I have hade in the past minimal pollination problems with zucchini, but this years the first 7 or 8 zucs all aborted. The difference? In the next bed I have 5 squash plants (butternut and some cinderella type winter squash) and those flowers are full of bumblebees (in fact, I tried to make a squash blossom omelette and I could not because every single flower was occupied). The zucs, nada.
Also, someone around here is doing something to the yellow jackets because I have not seen a single one this year.

Hand pollination has solved the problem for the time being (I use a small paint brush, that I push into a few males, and then into all the females), and just today I picked 5 of 6 hand pollinated zucs, and they are all fine. But I need to find a way to get the pollinators back in there.

Wait a sec... if it's "brain-like" are you sure they're males? It sounds like you're talking about the shape inside a female, and if that's the case, there won't be any pollen inside them. They also have a rubbery, sort of sticky texture inside to receive the male pollen.

If it looks like the flower in the picture, that's your answer as to why there's no pollen - it's a girl. ;)

Here is a link that might be useful: random website with a good picture

I finally uploaded a picture of my no-pollen male stamen onto flickr. It's definitely a boy as my girls are easy to identify by their large fruit even before they flower.
Could the soil be my culprit and just not have enough of something or too much of something else - resulting in many massive leaves instead of pollen production? I went out this morning at 8.00 and no pollen again on the new flowers...

http://www.flickr.com/photos/tealish/4767621479/

Oh interesting... THAT'S what you meant by "brain-like", lol. I was thinking maybe you were talking about all the nubbins clustered together inside a female. It's definitely strange looking with its green, shininess.

Andyandmone, I have exactly the same problem with my male zucchini flowers but have been hand polinating the females with pollen from other same-species (c. pepo) summer squash - pumpkin, acorn, spagetti, patty pan, etc. It works great if you have any of these available. I'm sorry I cannot answer your specific question though.

Also I've been told that male flowers with pollen can be refrigerated and used within the next day or two. Don't know about a 3-hr window unless it's regarding female viability.

I have the exact same problem. did you find out what it was??