My weather is seldom over 80°, so seed-set is seldom a real problem, but there are always a few plants which are difficult. This is a compilation of information I've gleaned from old-school hybridizers.
I have better luck with the tough customers if I move them under some afternoon shade, and keep them well watered. A saucer helps, and I never let it go dry until the pods are well on their way.
Some hybridizers swear by "starter pollen". With tetraploids, you can use diploid pollen in order to get them started. The dip pods will abort after a week or so, but the process seems to trigger hormones which get the plant "in the mood" for seed making.
There are tetraploids with very reliable and powerful pollen. I keep 'Atlanta Antique Satin" around because of this, and use it to get the difficult tetraploids going. AAS is an induced tetraploid—I think it came from a batch of seeds which were treated with colchicine—so it could either be producing very strong tetraploid pollen, or it may still produce a few grains of diploid pollen. I've never examined it with a microscope. My tet conversion of 'Homeward Bound' is the same way.
Of course, it's hard to know if the starter pollen is doing the job, or if, as Marilyn notes, the last few flowers on the scape just tend to set more reliably. I have noticed that if a scape has gone without setting pods until the last blossom or two, even if a pod is set, many times the scape will decline, and along with it, the pod. So on the real tough cases, I like to use all of the tricks, plus pollinate early and often, just so that if any interesting opportunities come along later, the scape will be well-set with pods and has a better chance of setting and maturing the seed.
Holding back on nitrogen is supposed to help too. You can fertilize normally early in the season to build plant size, but when scapes start coming, cut the feeding and just give them lots of water.
Another old trick is to break the plant into small divisions in the fall. Small divisions put up small scapes with few buds, but tend to set pods more easily.
You can also push the plants strongly though the early part of the season in order to get rebloom scapes, which tend to be much more fertile than primary scapes. It's as if the plant knows it's going to rebloom, and "saves its energy" for the second set of scapes.
