That's the whole idea, you have to get it back behind your head as the OHS is essentially the squat part of the snatch. With any significant weight you will lose it if it is directly overhead.
This is what is so great about the exercise, it tells you right away where you are inflexible. Dropping the bar because you can't get your shoulders back far enough? You need to work on shoulder flexibility. Can't maintain an upright torso at the bottom of the squat and keep tipping forward on to your toes? You need to look at your lower back and hip flexibility.
My aim is to get up to bodyweight (95kg) OHS for at least 5 reps by mid next year. At the moment I am not doing them as I am rehabilitating my shoulders with the AC and CW's Monster Shoulders program, but on my last program which focused on O-lifts I used OHS as a warmup for 3x3. By the last workout I was up to 67.5kg, so I still have a bit of work to do.
Just remember to build up slowly and work on form. If you find a flexibility issue work on it with stretching as well as doing OHS.