In my experience and reading through others experiences.. at least with Monoculus it takes a few tries for some to get it right Answer is They are suppose to be adhesive
Though This is not always the case it took mine 12 spawns to get the eggs sticking to the spot consistently the first couple spawns were completely slippery And stuck to NOTHING!!
There are still some that don't .. but those are mainly the female missing the spawning rock completely And I have foudn that now even most of those get fertilised some how.... Water flow