Firstly, for future reference, don't water the plants right after repotting, they need time to grow new roots and get used to the new soil and environment. And few other things:

1. It might be rot. Is the base of the plant mushy? You should probably check the roots...

2. The soil looks super heavy with peat and organic stuff. These plants need better drainage. If you can get your hands on some pumice, turface, and poultry grit then you can make even better stuff. But if you need a quick fix, try at least 50/50 soil and perlite mix.

3. Looks like you have a split rock Pleiospilos nelii in there...I would suggest digging that out and plant it by itself, it needs very different watering regime than your other plants. Good luck! More pictures of your whole pot and plants might help identifying more problems if any.

This post was edited by DavidL.ca on Fri, Aug 1, 14 at 2:56

You're not losing a plant; you've just added two more.

I suspect too much wafer and/or poor drainage. Cacti and succulents often drop leaves in response to too much water, and those pads look very turgid.

I would follow David's soil advice. Get them into something that drains really well and has no peat in it at all. Pumice, Al's gritty mix, even gravel, lava rock, or river stones will be better than normal potting soil.

In order to keep cacti longterm in peat mix, they must be in tiny pots with well-established root systems, then you only water when the plant shrivels. Bone dry is what cacti NEED. They suck up as much water as they can store it in their stems/leaves, and that's what they live on, not wafer at the roofs. When you water a turgid cactus, the water has nowhere to go and the roots just stand in it and rot. Fat cactus means do not water no matter how dry the soil is. In a home, most cacti can go many, many months without water.

What I meant by "adding two more" is that those pads will be lovely new cactus. Leave them sit in a warm, dry place for a week or so to callous, then stick them upright, calloused end down, into DRY well-draining mix and DO NOT WATER them. IGNORE them until you see new growth. Once the new growth is obvious, you can water them as long as you remember to always wait for don shriveling before watering again.

It is hard to teach ourselves that cacti don't want our love and TLC, but they really don't. Most cacti die of TLC. Pretend it's plastic. It will thank you for it.

Good luck with your two new cacti! :-)