you are not getting the cove in the correct position.
cut your self a template about 12inches to 24 inches long, with an internal angle on both ends. offer your length of cove into position, place the template against one internal angle, mark the wall and ceiling line with a pencil mark, repeat the other internal. you now know that you have got both internals in the correct position so fix that length of cove. work your way around the room using this method,
for externals, i cut both externals and butter and push them togther. you should never have to adjust your mitre.
as i have said in previous post, a weak mix of pva on ceiling and walls so we can move cove into its final position, also it stops the adhesive drying out to quick , and being dead. we would put a nail under,and over cove into the ceiling to prevent slipping.