Dying clematis

I have 2 new clematis plants which were happily growing up my pergola.  They have both now gone floppy at the top and the bottom leaves are turning brown and floppy.  There are a few little avids on the top leaves but I can't see anything else.  They are planted in new compost mixed with home made compost and some slow release plant food.  Does anyone have an idea on what's killing them?