I suspect that you end up with a few that are something like 50% elemental and maybe 30% Hit Fatigue.
Here's what we know. To get 100% elemental, you need a shadow hammer with powerful. To get max hit fatigue, you need copper and powerful (I think...since hit fat can't be imbued, we're not quite sure.)
It would stand to reason, then, that in order to get 100% elemental, and maybe 40% HFat, you'd need something like the combination of strength from a shadow hammer + powerful, and a copper hammer + powerful.
My gut says you're just not using a powerful enough hammer. I think you need to use Bronze + Powerful perhaps. The problem here is, that you're all but guaranteed a 3rd property. Now, assuming that you'd want the sword to have something like HML/HLL/HSL too, this isn't an issue - get one of these and then just imbue over the top of it. The problem comes from the numbers.
The odds of getting something that you really like: 100% elemental, 40+% HFat, +small amount of some 3rd prop you wanted anyway get really small. It may still take you a ton of charges, but now of bigger hammers. Even if Coppers could do it, the logic says that if you get 1 HFat with 50% intensity per 10 tries, and you get 1 weapon at 100% elemental per 10 tries, then getting 1 with HFat + 100% elemental should be roughtly 1 in 100. Since you're using 6 charges per use, having burned ~800 charges doesn't seem crazy.