I phone 4 and Imac Icloud text issue

I have an imac and most people that text me on my iphone 4 comes through on my imac via icloud. It's still working well for all my contacts except one and it comes up red. So when i recieve texts from this person on my phone I cannot see it or respond on my imac, and if I try to text them on my Imac it shows undeliverable?? Help