Fivealive, my trout experiences are similar. I've added 10 to 20 RNBT, BRNT, and TT (acronym for tiger trout?) for several years. After some harvest, there then may only be 1, 2, or 3 left alive and on 1 or 2 floaters. I don't know where they go. Osprey? Eagles? Dead and simply sink into the weeds for crayfish food? Many dead fish never float, but you probably already know that.
Do you feed? Surface pellet eating rainbows are simple to fool with something that looks like what they've been eating.
You might try live bait. If you have crayfish in the pond, try a small one suspended. Trout just are not that smart, but they can be finicky.
Good luck, and keep us in the loop.
