I think there are different reasons for different plants. In my experience, if you can 'pop' the leaf off the plant without tearing flesh, the leaf will sprout. If the leaf has to be cut or torn away, this plant needs an intact head to start. Haworthia, aloe and senecio are good examples of non-leaf starters. Sometimes the new leaves will develop first and sometimes the roots will develop first. Some leaves prefer to be partially buried and will take much longer to sprout up.

Full sun will burn leaf starts. No sun doesn't work that well either.
Also, some leaves will start, others of the same plant will not. It's a numbers game.

I can't tell you too much about cactus.

Haworthia do start from leaves, but that takes some experience, and proper mix. They are winter growers. Grow in low light, so are good houseplants, but really do not like the heat so these can be grown on shady window sills.
Aloe will not start from a leaf but an offset, it will also grow it you cut off the head and leave just a little stem and set it on top of the soil when the weather is warm, it will root itself, do this in July. I don't think you have the right soil mix. for your climate. Only 10% organic in the mix, no sand, water can't get through it, try 60% pumice or perlite 30% coarse washed construction sand, it must have fast drainage, and the plants must have light they also need air,the may heater kill them. When watering don't let them sit in left over water that is in a saucer. Check with a chop stick to be sure the water is reaching the root ball, and not just going down the sides. Judge the size of the pot to the size of the roots, even some Haworthia need pots 6" deep not wide. What is 'Ripple Jade" parden me, but that is a new name for me, this plant will do well with poor soils with lots of rocks in it. Little water very good light. Water only once a month and let them crowd the pot then transfer to a larger pot, until they out grow it, and repeat the process with new fresh mix. The Kalanchoe if you cut off the top, leaving only a few leaves, keep the plant warm and good sun, will grow new offsets from the top of the stem. Peanut Cactus need morning and So.facing sun, I put mine in shade for the afternoon, my sun here is far to hot. Any plant from Africa are usually Winter growers, and grow on rock out crops, poor soil, and very lttle rain water, except in the So. Cape. Senecio need warmth amd be found with the name tomentosus in Hort. So Africa Cape. it was pouring in Sept. It is a summer grower in this country. When you take a cutting let it dry out for at least 3 days in light shade, make sure it's getting air flow. Plants need fresh air just like we do, food, water, love, knowing when they are sick, and taking care of them. If a plant looks sick or a cutting take it out of the potting mix perhaps take a new cut and let it dry out again, pay attention to the proper time to take the cutting, a general rule is after they flower they begin to grow again, give a plant light fertilizer like Osmocote until you get the hang of fertilizer. You can't burn them with this product, again only fertilize during their growing season, don't fertilize a plant out of season. I have propagated plants for the past 20 years I actually started as a child, many plants you can start from leaves, seeds, cuts and some Haworthis , Sansevieria by just the roots, some just by a leaf, the trick is knowing which one is which, you will make mistakes just like I did, but that is how I learned. If you don't try you will never know, continue to ask questions, this is how we all learn. I even began as a child to grow green thumbs, LOL don't give up you will find the hobby very rewarding. I can't say this is sure fire advise, I don't know your growing conditions these are suggestions and not written in stone, this is just the way I do it in my hot dry valley which is very much like Africa and our South West. Again these instructions you may not need I don't know if you are a beginner or not, and don't want to offend you or any one else. Norma

Thanks for the advice, both of you. I'm a beginner so I'm not offended. I'm wondering if some of the cuttings were getting TOO much light at first and getting shocked.

'Ripple' was the name on the plant - got it at HD I think. According to this site:

56954 CRASSULA OVATA RIPPLE JADE CRA HP CGH PRICE: $9.50 'Ripple Leaf Jade.' Rather unusual new hybrid of Jade with thinner leaves that undulate along the edges and have a powdery tomentus on the leaf. Obviously needs a bright window or under lights. We are not sure of the parentage of this yet. It might have both Crassula arborescens and Crassula ovata in its blood.

Here is a link that might be useful: scroll down for CRASSULA OVATA RIPPLE JADE

One thing people tend to overlook is cleanliness... soak used pots in 15% bleach for about 15 minutes to kill any bad microbes that may live there. Also make sure you use only fresh unused soil for little ones. Take the used soil and have an area for it. if your like me then you will be suprised at what came up in that area. A good fungicide such as Dithane is a must. One thing I have learned over the years is that when the temperature drops below 70 or extremely humid for for any period of time that fungus and bacteria are present in abundance. What you describe seems to be more on the fungus side. Also, try different rooting hormones to see if you get any better success.

That Jade might also be called "Whirlybird" - a cool Jade, indeed.

Josh