I have bought two lemongrass plants, which I have potted with John Innes No 2 but the leaves are going light yes slow..is this the
Certified GKH Gardening Expert
The compost is fine... It needs to be mixed with potting soil, though. It doesn't appear to have any aeration, and it is likely much too strong for a grass. I would mix it into a potting mix at 1/3 strength, and work from there, adding a handful every few weeks.
This article will give you more information on the care of lemongrass: https://www.gardeningknowhow.com/edible/herbs/lemongrass/learn-about-growing-lemon-grass-plant.htm