What method are you using to clone? I give up on mine after three weeks. If the clone looses it's turgidity (firmness, water pressure) then it's probably done. Clones that are about to pop roots usually perk up a bit just before. Some yellowing of the lower leaves is OK and not an indication that the clone is dying. Yellowing leaves often means the plant is using energy to make roots, but can sometimes mean your light source is too strong.