I grow mine in John Innes No 2 (or use a loamless compost) in full light but shade from hot sun and they have good all round ventilation. I water sparingly about twice a week during the growing season and feed with a high-potash feed every week.
Hope this helps?

Looks like it needs a decent feed to me, something general purpose to get some nitrogen into it. This particular form of Pelargonium does not appreciate full sun, likes a bit of shade, so half and half is about right. Feed with something like Miracle Gro general purpose.

Many thanks for the suggestins. As it happens, I've got some Miracle Gro, so I'll try that.