Maybe a cage like trellis or a couple more strings would help a little.

Compost and mulch maybe? That would help mitigate any mineral deficiencies and stabilize soil moisture.

i think you might be right with drought, we had a very wet year and quite cool in england and my passionfruits are growing very well.

That's strange because they are very low maintenance plants. Maybe being by a walk way means there could potentially be salt from deicing the paths in winter or herbicide, that doesn't look like herbicide or salt damage. I don't think lack of water is the cause on its own because that too should look like what you'd see from herbicide. Since you said it's a south facing, excess heat from that wall they're planted by could be essentially cooking the plants on very hot days. I think that it's most likely that. I'd try moving them to something they can climb that doesn't absorb, retain, and radiate so much heat. A freestanding trellis would be good, or an east or west facing wall if you want it to still be on a wall.

Roots and moisture - Roots may not be able to penetrate deep as you have against wall and the foundations may stick out - leaving shallow bed. Yes, also the concrete (if not waterproofed) will suck the moisture out of the soil. Finally it could be contaminants in soil - maybe passion fruit is sensitive to aluminium in concrete.

You are right about your assumption that it is too dry there. Put down 4” of mulch across that whole strip and it should prevent enough of the water from evaporating to keep them happy.

I had two passion fruit not survived last winter, and all 10 of my party kiwis took two years before they started to do anything other than looking dead, I have two more replacement passion fruits, but I've heard that they are quite aggressive when they survive.

My guess would be alkaline soil and/or needing more water. It's not where the chem students dump their extra RoundUp is it? ;) My brief research says they like the 6.5 ph, like basically everything besides blueberries do, but there was a research paper saying they do well with very acidic soil. Water and some acidifying fertilizer or sulfur pellets is what I'd go with.

It is still early, my passion vines (p. incarnata) are only 30-50cm. I think yours would benefit from thick mulch and water. Maybe check the PH, it might be too alkaline from the concrete and stucco.

I have had several young p. edulis die seemingly from waterlogged soil. p. caerulea is an illegal invasive here so I couldn't comment on that one.

They will do fine if your weather stays above 18C for the growing season. The only mistake you made is not planting them in a container. For me in zone 6b (newly changed to 7a) growing passiflora incarnata, passiflora lutea, and passiflora edulis, the first year they establish their roots. The second year they have a few flowers with maybe a couple fruit, and the third year you will have dozens of fruit…because the roots send up new shoots 10ft away in all directions. I corrected that mistake by burying large pots with drainage. I found roots the width of my fingers about 18” deep so do with that info what you will 😂. Oh, and I have my edulis in pots I bring indoors over winter because it gets down to -26C here in the Midwest USA. My basement stays around 18C all winter and I put them in front of a large window and they do fine.

I have a lot of plants against a retaining wall that has backfill to stabalize the wall and they are fruiting (not passionfruit but apple, pomegranate, fig, etc) so it's not an issue that there is rubble the roots will find a way. If it's dry, add mulch but dont cover the stem so it doesn't rot. Water with a water soluable nitrogen fertilizer in the watering can or add some feeding pellets so when it does rain it will release some. Nitrogen will push new green leaves to grow out.

I had a couple of incarnata growing in my garden in Denmark last year, their first year, and they had a lot of growth and flowers. This year they did not come up and I thought they were dead - but a couple of days ago, I saw one emerging from the ground. They are growing in light shade in a place that never gets bone dry. I do water sometimes. This year, I have bought a couple more incarnatas and in addition to that edulis, as I want fruits. I have tried new locations, still light shade. But snails eat them almost to the ground.. as it seems to be their favourite plant in this wet sommer with lots of them. So snails seem to be the biggest problem here now, and plants do well in light shade in not too dry conditions.

Might be lack of water but also young passionfruits don't compete well with weeds ( which also compete for water ). I'd be tempted to weed that bed then mulch it

No one I know has had good luck growing them around here, Seattle WA, probably due to root rot from being too wet in winter.

They look awful, definitely should start an intensive regime to water and fertilise, A LOT, as well as weeding and mulching. Sometimes when planting so close to a building there can be a lack of water in the soil even if it rains, especially when the prevailing wind is blowing from behind the building and a rain-shadow is created. Also like has been said you need a trellis style support to grow passionvines, they're not runner beans and need to be able to spread horizontally too, by far most of the growth will be horizontal. It almost looks like it's got a severe sap sucking insect problem but maybe it's just the poor conditions.

Wrong country to plant it?

Fertilizer