Help! My Virginia Creeper is wilting

Hi there

Hope someone can help. I have a 3-year old virginia creeper climbing up a north-facing wall, and it has begun to wilt badly.

It's only the end of May, and new shoots are still appearing, but two or three of the main established stems all have completely wilted leaves (which have not long opened.) Other stems/leaves look fine, and there is no sign of any aphid or bugs.

Can anyone suggest what is happening, and what can be done?