Dreads can sometimes need repair. They can become weak overtime for a variety of reasons. Sometimes its because they were made poorly to begin with. Because they have been styled too tightly and too frequently. Or simply because the hair has grown weak as the hair has matured or you have aged.
If the dreads are thinning for whatever reason, it's possible to join adjacent dreads together to create a stronger one. This can normally be done with the surrounding loose hair but occasionally extension hair can be added to help the bond.
If the dreads are too thick its possible to split the dread as make 2. This is not simply splicing the dread, its skilled and delicate work.
If the dreads have become weak due to hair loss or alopecia, there are also techniques to help reduce the fallout and ways to style the hair to hide the loss and be gentler on the hair.