A difficult Rex

I have propagated begonias in the past, but I'm having trouble with this Rex. I took a leaf cutting (with petiole), applied rooting hormone to the cut, also made cuts along the veins, and inserted in moist medium on 1/13. Kept it loosely covered for a week to retain moisture and it did well for awhile, but then the whole leaf started to rot, so I dumped it - never grew any roots. Took a second cutting (leaf with petiole) and put it in water on 2/13. It did develop some roots on the petiole and I potted it up on 2/22. Now that leaf is "wilting" and turning brown around the edges. There is no sign of any new growth yet. I know the cuttings take awhile, but my leaves seem to die out before the new growth starts. Any suggestions?

Keep it covered, don't keep it too wet (it will rot), and be patient - if it has roots then it should throw some babies up soon (brown edges don't matter but a brown center is disaster).

I've had good results using the larger clear plastic "salad greens" containers from the grocery store. They're large enough to place the entire leaf (major veins cut) onto fresh, moist potting soil surface; the lid keeps moisture in (must be careful not to over water). The container is placed in a warm area in bright indirect light. If moisture collects on the lid, remove and shake it off before replacing. I occasionally check the soil surface, spritzing with water if dry.

Well, I'm on my 3rd leaf cutting - the other 2 rotted, even the one that had rooted in water! This one was carefully set on moist soil and covered loosely with plastic. After only a few days, it appears that it also is turning brown at the center :( And I was very careful not to wet the leaf this time (think I overwatered the other ones). Somehow, this plant just won't propagate for me!