Losing the passion...flower!

Hi everyone. I have a rather sad looking passion flower and wonder if you can help? See the pictures below...

About three weeks ago...

Today...

The panel on the left has one passion flower plant and seems to be doing very well. The panel on the right has another plant and as you can see it is losing its leaves and the flower buds are shrivelling up. It was fine until about a week ago when it started to wilt. One of the vines on the plant now has almost no leaves at all and the fronds are drooping down. The other vine of the same plant has leaves but these are starting to turn lighter geen and droop as the others did.

I've checked the roots and there's no sign of vine weevils. The leaves that are still there are not showing signs of virus. The stems are intact and have not been eaten away. 

They're both in good soil and have been fed and watered in the same way regularly. It's sad to see one struggle after having made such a good start after they were both planted as 60cm or so tall plants about three months ago.

So basically, I'm keen to know if I'm doing something wrong or is there something that can be done to bring it back to life?

Any suggestions or ideas would be much appreciated!

Thanks in advance, Philip.