Try increasing your calorie intake so that you are gaining 0.5kg a week. Approx. 1lb per week. Eat every 2 hours, have low GI carbs (Brown bread, pasta, rice, oats) and high GI carbs after workout and in the morning (cereal can be quite sugary, maltodextrin and glucose after workout is vital imho). Approx. 35-40g protein in each meal, only use shakes in the morning and post workout, try to eat eggs, dairy, fish and chicken for protein. Fats should be around 10-15g per meal BUT do not have them just before workout or after workout, you need quick digestion at that time. I'm also not a fan of fats in the morning, at night they work wonders. Make sure your getting 8-10 hours sleep and really straining your body. Maintain this and its physically impossible not to gain.
IcE