If I understand you correctly, you are talking about the ‘hairpin’ smart shape crescendo - right?

The crescendo hairpin can not play back by itself.

Instead, it is Human Playback that interprets the hairpin.

I would check the Human Playback settings.

If I begin the harp with a dynamic marking  and end it with a dynamic marking it always plays back a cresendo even later when I make a MP3 file. What is happening now is It will disapear from the score on play back and not come back. It has to be re entered into the score to even be seen.

If Peter's suggestion does not work (which would be rare in and of itself) be sure your dynamic is applied to an active layer.

I must admit I have not yet experienced your situation.