Your results sound a bit extreme. I've used the Jiffy 7 pellets with some success, but prefer using the 2.25" x 2.25" square peat pots with a fine textured potting soil. Although I don't clone very often, my success rate usually exceeds 80%.
When I take cuttings, I immediately submerge them in a clean bucket of cool(not cold) tap water. This allows the cuts to maximize their water content. I trim each cutting while submerged. I treat the trimmed cuts with the rooting compounds and then place them into the peat pots, making sure the stem reaches the bottom of the pot. Do each cut quickly!
Most importantly:
NEVER LET THE CUTS WILT!(If it wilts, it isn't getting enough water. Water more, or remove a leaf or two.)
A cutting must be able to support itself while making roots. Temperatures in the 75-80 degree range are best, in my experience. High humidity, tapering off over a month or so(less if the cuts begin to show new leaves, sooner). Good low temperature lighting. The more, the better.(heat kills clones, not light.)
Your hood sounded good. I usually offset it about an inch to allow CO2 rich air to slowly circulate.
Mist frequently to prevent drying, tapering off with the humidity.
I use Olivia's Rooting Gel and usually dip that in Rootone powder. As far as I know, Rootone is the only rooting product that contains a fungicide. I credit THAT with my continued cloning success. My Grandmother told me about Rootone, 25 years ago.(She is responsible for teaching me to garden, successfully, starting at age three. I was cloning Coleus and African Violets by the age of eight.)
A few no-no's.
1) Never fertilize unrooted cuttings! Once roots begin to show use a liquid fertilizer at 1/4 strength every other watering.
2) NEVER let the rooting medium dry out.
3) After the first two days, don't allow water to stand around the pellets/pots/rockwool cubes.
4) Don't leave too many leaves on each cut. Remember, the cut is getting water from stem tissue and leaves(if you are misting). You must balance the plant's ability to uptake water against leaf evaporation.
5) The cut will draw energy from its larger sun leaves. These will turn yellow and then die. LEAVE them until they are obviously used up and brown.
Once you see new leaf growth, the plant has roots. Move to a larger container, but water regularly. Not all rooted clones are able to support themselves, right away. They still need a bit of TLC.
If you don't think this addressed your problem, tell me more. We'll get it figured out.
A few pix on a sunny winter day...