Help with cuttings

Not sure if this is the right spot for this, but let’s try! I’ve been trying to root some cuttings (rooting cuttings is the hardest part for me!!) and I’ve had a wave of failures, generally one or two a day around the 4 week mark. They get a yellow tinge halfway up the stem, then a couple of days later they go blotchy brown. When I pull them, they have a nice callus, but failed to move beyond that. Any advice would be appreciated. Them dying in the middle first is what gets me.