hi everyone very new to gardening, iam taking cuttings from dahlias that have been producing new growth in a propagator, the cutting are about 3 to 4 inches cut low down to the tuber, i have put them in a mix of grit,multipurpose and perlite, i have covered them with a plastic bag, but after a few days they have gone black and dissapered, is it to early , i have put them uncovered in the propogator, but the same has happened, so experts can you suggest what iam doing wrong ?