Clone slowly getting worse yellow and browning

[Stanknugs]

Cut this clone along with 4 others from various plants. This particular clone i cut from a plant that has a bunch of yellowing as well. Any seasoned growers have any suggestions on how to fix this. Feeding clone with 6.0 ph water.
Plant is getting blue planet nutes.
Here is the plant the clone came from.

 

[WizHigh]

The rockwool cube isnt tight enough around the stem. Tighten the cube around the stem by either squeezing it together or using a twisty tie. A twisty tie works better. Plus you need a dome over top of the plant. Either a plastic cup or a sandwich bag will do. Also feed with B-1(super thrive/ thrive alive)