Young plants with yellow growing tips

[mancorn]

Transplanted some seedlings 7 days ago into a FoxFarm mix of Happy Frog & Ocean Forest and all the growing tips are yellowing. Noticed the first one a day or so after transplanting and it's spread to 6 out of 8 plants (different strains). No nutrients and pH is mid 6. Seen an aphid, but nothing else. Any suggestions?

This is the first one that showed signs and the yellow in a little raised (bubble).

 

[Jack og]

It’s fine, new growth starts light and darkens in about a few days but they need a good veg feed soon