Lower leaves that yellow could be a sign of too much water but your hibiscus look healthy except for the small amount of yellowing. From the image you sent it looks like it could just be transplant shock and a slight adjustment as the plants adjust to full sun after a more lush life at the nursery. Wait a few more weeks and I suspect the plants will be in bud and flower and looking better. Meanwhile you can remove any yellow foliage. Keep growing, Marianne Binetti