I have painted my landing doors, frames, coving and skirting board twice, as the first time I painted it it turned yellow. It has now turned yellow again, but not all of it. It was all painted at the same time with the same paint. Same of the skirting board is still white while other sections have turned yellow. I also painted the coving in my bedroom which has also turned yellow. I understand that lack of sun light can effect the paint. My bedroom has a lot of sunlight, so I don't understand why it has turned yellow or why some of the paint on my landing has turned yellow but not all of it. The landing has the same amount of light all round. Do I have to paint the whole thing again for the third time, and if so what paint should I use. On the previous occasions I used Dulux Gloss pure white paint. Please help!