If the ball is too small in diameter it can "lock" in the bezel slots. If the spring is too long it canpush the bezel up at an angle and cause it to be very stiff.

If your bezel (with seal) turns freely without too much wobble when no ball/spring is fitted it should only be a matter of the right sized ball and spring pressure.

 

I also forgot to mention the spring. If it's too long it could become coil bound (can't compress the required distance).
With the bezel removed, check that the ball protrudes just less than half its diameter above the surface of the case.
Take a soft metal or plastic stick and press the ball down, it should be able to be pressed down to just below the surface of the case.

If it won't go fully down you may have a bound spring, or the ball may be larger in diameter than the hole, in that case, the hole would need to be drilled to suit the diameter of the ball.

 

Thanks for your prompt response, Jim, and apologies for my rather late one. I had checked back but failed to see your reply.

Sounds exactly right. I did wonder about the diameter of the ball, because it seems to sit quite high. I'll have a look at your suggestions and let you know the outcome.

Thanks again!