patchy ceilings

Recently, when rolling large, flat ceilings I've noticed the johnstones cova plus can dry quite patchy. Any one else had this problem, is it a paint or application problem? Most of the time you can't see it from floor level, only when you're up a pair of steps, looking across it, although occasionally is visible from floor level with particularly large ceilings.
Edit - only seems to be when using brilliant white.