Are they planted on top of the landscape fabric?

yes, they are... should've realized this might be the problem. I guess I just figured that they would penetrate still because it is fairly thin fabric and the weeds still get through sometimes!

The Sedum would probably be OK instead of great, but overall, you're setting up a really frustrating situation down the road.

Cardboard under mulch is a much more reasonable solution. It decomposes, roots do not get tangled up in it, and can be replaced as necessary. Everything said about the benefits of landscape fabric is a lie, when one considers a spot 2+ years after laying it down, in a permanent planting.

As organic matter such as leaves and mulch decompose on top of landscape fabric, and grit accumulates when the wind blows, seeds are given a place to germinate, just as if the weed fabric wasn't there at all. Trying to remove weeds after that is difficult because the roots do become tangled in the fabric, as well as roots from desirable plants tangling in it from underneath. Sedum is a xeric plant, but still can't perform at anywhere near its' potential on landscape fabric. Most plants would need a hole through the landscape fabric to get many more roots more deeply into the soil. The hole also gives weed seeds/roots an opportunity to go through. Since it is unable to contact the ground directly, the mulch layer on top of landscape fabric will decompose much more slowly, nullifying/slowing much of its' benefit to plants in that spot.

Both adding plants and pulling weeds can cause the landscape fabric to come to the top, so it usually looks strange, with black wrinkles in a flower bed.

First thing, get rid of the landscape fabric. As said above, it doesn't work for very long. Your Sedums and all other plants you have will appreciate it. Invest your money in a good mulch and ground covers to take over when the mulch decays.
Sedums can grow where other plants won't so that's where I plant them to avoid weeding them. Grass is one of the worst weeds for Sedums here, so I don't plant Sedums where grass will grow. Problem solved.
Here's some Sedums growing on a stump in my rural, informal garden.
Mike
{{gwi:687}}

Here is a link that might be useful: My garden pics