Dahlia propagation

So Im back taking dahlia cuttings. The cuttings seem to be very vigorous so I'm trying again, they really fill up your garden.Just need the boss to visit to dig me a few more beds . I thought I'd cracked it last year but I think the humidity changes as the months go on and obviously the warmer it gets the quicker they root.
So far Ive lost quite a few while on the heat mat - the watering of the heatmat to make sure the heat is even makes the compost too wet for the cutting? So I have tried cuttings potted up in the same open/gritty mix and in closed plastic bags, which seems to work well. When the cutting material starts to speed up a bit I am going to set another heated propagator up with a sand on the base - no lid. Hoping this improves my early success.