Mine have survived this Tennessee drought we've got going on down here with only being watered every 3-5 days and they're trained against a tree as their trellis for this year.

They're actually doing a lot better than I thought they would because the area is bright shade until about 3:00PM and then they get full sun for three hours and then back to bright shade until sunset a couple hours later.

What about your other conditions, drainage, nutrients, watering, etc.? How much heat emanates off that wall during the day, is it possible that's "baking" them until they finally wither and die off?

I have moon vines growing and blooming everywhere but the ones in full sun are doing the best. I had 8 blooms on this vine tonight and it is loaded with buds. I have found that they do not transplant well. I start mine in 4" pots and plant when they get about 6" tall. I also am growing some purples this year so I am hopeful for seeds from these. I do not think that heat bothers them as long as they get enough water. I live in Louisiana and we have very hot temps and high humidity and they still grow to amazing sizes here.