Drooping blueberry leaves

I have some 3 year old blueberries in pots, all with ericaceous compost which I topped up earlier this year.  One of them has leaves which have started to droop.  I checked the compost, and it's moist but not too damp.  I only ever water them with rainwater.  Any ideas on what's causing the leaves to wilt please?  I took out the top 6 inches of compost earlier and topped it up with more ericaceous compost just in case it is lacking nutrients.  The other 2 plants are fine and all have been treated and watered the same way.

Any suggestions gratefully received.

Thanks

Potters