Regular potting mix can be problematic. Too wet and everything rots and mold develops. Too dry and cuttings eventually dry out.

Perlite is usually the easiest thing to root in. Remember to put in drain holes and cover your cuttings.

I've rooted rexes in straight potting mix (Miracle-Gro or Jungle Growth) with no water - the mix was already moist. Make sure you cover your cuttings. I prefer roasting pans with plastic clear lids. Poke holes in the bottom and fill with media (perlite, potting mix, combo, sand, gravel, Nature's Helper, etc) Some cuttings do fine by placing the leaf flat on the surface while others do better if you prop the leaf vertically and just have the stem inserted.

Water rooting is best with the entire leaf instead of wedges but then I haven't tried wedges that much with water rooting. Also cover your cuttings even if you are doing it in water. I've had water rooted begonias in the same cup of water for up to 3 months inside a cookie jar for example. If let out in air you must monitor the conditions of your water and cutting daily. I suspect that bacteria and other germs are multiplying rapidly under normal conditions but don't know why it stays so clean when put into a terrarium like environment.

The other thing is to select good size healthy leaves for best results. Time of year is another parameter. Summer and fall might be better for rexes.

Anyway, try, try again and put a cover on your cuttings.