Some plants are just easier to root than others. Different species of the same genera can have varying rates of success. I've not tried softwood cuttings, but HERE is a site that gives a basic description of taking softwood cuttings. You should put each cutting in a plastic bag to increase humidity. Other than that you are doing it right. Good luck.

After re-reading your post you may already be using the top half of the juice container as a humidity tent. If so, I've nothing more to add.

I cut the large juice container in Half except for a "hinge". I poke holes in the bottom then a few holes in the top then I tape them together with duct tape after I am done with putting the cuttings in the perlite inside it.. When I see a good deal of roots through the clear plastic; I then know that there has been success. This has worked for salvias; mexican oregano; Ageratina havanese; lavandar; Turks Cap. Yes the top remains attached to the bottom.

I used the whole branch from the semi hard to the very soft. Whatever makes it will make it.