Are you asking about a Buddleia and is it a seedling? Because I'm finding it difficult to work out what on earth you're asking us if its not that.

If it is yellowing leaves then the plant needs some added nutrients. half strength tomato food, or growmore type feed.

Or possibly just water, its bone dry here in the south, Seaburng.

yes that would do it. I just assumed that the basic needs are being given.