any ideas my good friends?

As i have posted a few threads about my new seedlings and appreciate all the comments and help i have recieved.However i have noticed a prob with 4 of my plants(i have 15) the last set of new leaves on these 4 are turning yellow in a strange way.The very tips are still green and round about the edge,but in the middle is turning yellow from what i have managed to research in a nitrogen prob the tips turn yellow then the whole leave(i may be wrong)all the bottom leaves are a nice green.They have just been repotted and put into my grow room.They are in bio bizz all mix and getting bio bizz grow.Has anyone had a similar problem?Also is there anything i can buy to remedy this prob?I'm thinking maybe an iron defiency but to be onest i am not an expierenced grower really.Could i maybe use a bit of my bio bizz bloom they are due to be fed tomorrow.I am starting to worry cz the seeds cost a fair bit and i have fussed over them for 7weeks now and even though they look really good these 4 are causing me a bit of concern.Wish i could post a pic but it's not posible at the moment.

Looking through the plant problem guide, you may be right about the iron deficiency. I just don't see any other pics that match your description.

Thanks for taking the time to answer my post.I will get an iron remedy.