Thank you for your question. It would be useful for us to get more information on maintenance and site, but here are some plausible options. It appears that you have mulch down on the bed. Ground covers can typically replace mulch, making this unnecessary. It's possible that you are actually getting too much water with the mulch down. Well established sedum can survive very little water, so without knowing your watering routine, too much water seems more likely than too little water.

It could also be the natural result of overgrowth/old growth. One thing you could try regardless is to remove those dead and dying stems and either wait for the sedum to spread, or take cuttings of the healthy plants and replant them in the bare space. Methods for propagating succulents through cuttings are outlined in the link below.

https://www.ndsu.edu/pubweb/chiwonlee/plsc368/student/papers01/pneisen/Propagationofcactiandsucculen...