Could be something with the acceleration / deceleration properties of the platform behaviour, which you can modify on the properties tab. Deceleration 0 will keep the sprite moving forever, 99999999(...) will stop it dry. Otherwise, you'll have to upload us a capx tu see...
I just got down exporting and uploading the game, It should be on my profile shortly, if you'd like, you can look for yourself with the issue may be