This could be a problem with collision detection, or a problem with your gem counting process or a problem with your HUD text.
To narrow this down, first change your SpriteText+ HUD to native Sprite Font or to a simple Text object. If this fixes the issue, check your SpriteFont+ settings.
If this still doesn't work, add a short sound playback to your "On collision" event, to test if it's triggered.
If you collide with a gem and there is no sound, then you'll know that you need to check collision settings/polygons etc.
If the sound is playing, then there is probably something wrong with how you are counting gems.