You didn't mention the type of coupler so i'll assume it's a knuckle coupler.
Set your train on a level track and look at that coupler set to see that they are aligned in height. If they're not Kadee sells knuckle couplers in 3 styles:
Over set which lowers the coupler
Center set which you probably have
Under set which raises the coupler
If the car is coming uncoupled due to uneven grades in the track you might want to use a shelf coupler.
http://www.kadee.com/index.shtml
Have fun
Set your train on a level track and look at that coupler set to see that they are aligned in height. If they're not Kadee sells knuckle couplers in 3 styles:
Over set which lowers the coupler
Center set which you probably have
Under set which raises the coupler
If the car is coming uncoupled due to uneven grades in the track you might want to use a shelf coupler.
http://www.kadee.com/index.shtml
Have fun