Joey, stop adding them, if they are being eaten you're throwing your money away, if they aren't, you're going to end up with a tank full of BB gobies.
They must be in the substrate, there aren't too many fish that will inhale 2 gobies at a time, and there aren't any in your tank.
If you want to find them, remove all decoarations and put them aside, then start moving the gravel around (gently) you can do this by waving your hand gently above the gravel(it will blow around).. you'll probably find you can uncover 1 or 2 easily.
Also, they are brackish, so you should be adding some salt to the water.