Hi Elizabeth, thanks for the question.

This could be a nutrient problem, water problem, or even a disease problem.  The green leaves show a lot of red which would indicate a problem with phosphorus uptake (availability or cold soil) Are the ones yellowing a different variety?  Are these inside or outside? If inside , how do you provide the light?   How old are these plants?  Were the soil blocks fertilized, if so with what/how much?  Have they been subject to cold temperatures?  To be safe, it would be a good idea to isolate the yellow ones. Sorry for all the questions.

I look forward to hearing back from you.