I use no descrimination, I adjust sensitivity till the unit is stable then make it just a tad hotter, volume is maxed out........A repeatable signal is a signal from 2 different directions that does not have a null.....With a null you may only get a signal from one direction.....
Those lost signals could easily be tiny pieces of aluminum trash that wash through your scoop, then float away in the current once they are released while digging...
Do you tune your excal before you hunt? Try this sometime...
Take a gold ring and tie a piece of string to it and bury it in the wet sand about 10-12 inches deep. ( for 10 inch coil) I dont know what coil your using, the larger the coil the deeper you try bury the ring..... Now try adjusting your settings till you can pick up the faint signal of the gold ring.....Don't just adjust the sensitivity, but adjust all the settings except the descrimination, basically tune in the gold ring....If you can't hear it at all, then raise the depth of the ring slightly and tune again till you can hear it.........