When Dolly was conceived, the chromosomal data inserted into the denucleated ovum was 'aged', had used many of the telomeres leaving less for a full lifetime than if the sheep had been conceived from original male and female gametes.
Also, within the cell are mitochondrial grains which contain genetic material unique to the female's ancestry but not the 'drivers' of growth and development for the ages the conceptus goes through. The uneven timeline is the problematic issue with cloning at present, though an enzyme, Telemerase, which re-starts the clock of telomeres has been discovered. Until ALL problems are solved with higher non-human mammals, no human reproductive cloning should be legal, for starters.
I hope that helps. There is a bit more detail in the essay posted on my profile age, if anyone is interested.