Panning can be very difficult to master. It comes down to muscle memory and practice. After a while it becomes second nature and then it’s easy. Now one would think that if you get used to a heavier lens, like the 200-400 then a lighter lens like a 24-70 would be easier, but it’s not. It’s just as hard because of that muscle memory. Then there comes the angle. If you think you have it down pat try shooting at a different height, for instance crouching down. Not only does this make for a unique vantage point between the foreground and the background it also will really challenge your skills.