You did not say what specific type of clown you have. Most clowns are very easy to pair up. Some take more patience than others. Tomato and maroon clowns are more fussy. Putting a divider in the tank can really help. Keep it there until the fish do not charge at each other, but hang out together. This can take a few days to weeks. You almost always want the new action to be a fair bit smaller then the established fish. Egg crate works well unless one is small enough to get through it.
Unfortunately it sounds like your not getting the healthiest fish or there is an issue with your tank.You should not be losing so many clowns, as they are pretty hardy.