50/50 perlite/soil mix works just fine. problem, if it's water, is with how much you watered them. in this case, how much water has been taken into soil by the sip set up. lookeing at the pix, the cutting still looks good. the whole length of the cutting is no drying out... yet. if this is problem with water, the cutting itself should dry out, not just the top end. looks more like lack of moisture dried out the bud.
two choices if it was my cutting. drastic measure is to pull it out and check the root. simple test is to cup the top to see if added humidity on top will bring back the bud.
if the whole length of the cutting is drying out, that means you cutting under the soil level has rotted.