One of the most important things in determining cutting success in these junipers is the health of the parent plant. I've done cuttings from hundreds of different plants, and if you get all the aftercare and everything else right, the percentages always reflect how vigorous the parent was at the time the cuttings were taken.
Hence, taking cuttings after a tough repotting and when you start seeing dieback is not going to yield the kind of success that you might hope for.
The size of your cuttings looks about right.
A finer, more water-retentive substrate would yield higher percentage success. Coarse bonsai soil is not ideal. I use perlite and coco coir, but even just potting soil works fine.
Keep out of direct sunlight in a shaded place for at least 6 weeks.
August is not an ideal time, I've seen 20% take in August while 90% from the same plant and same technique might take in October. Your location and weather will affect this though, as I'm doing the aftercare either outside in San Francisco, or in a greenhouse in San Francisco.
I don't see any mention or evidence of rooting hormone - while you can get some success without, you get more and more reliable rooting with an 8,000ppm powder or similar, I use Hormex #8, but soaking in a DipN'Grow solution or CLonex or many others will work fine.
Sanitize all materials to avoid transmitting surface pathogens from parent plant to propagated plant - I dip in ZeroTol.
Good luck!
E.
