How often are you doing water changes and how much? That was my #1 reason for my females constantly dropping eggs. When I stopped monkeying with the water and sticking with mainly topoffs (only changing 10-15% when TDS gets over 200) they all berried up rapidly. This was after 4 months of constantly watching dropped eggs. Oh, and they eat the eggs when they are dropped. If you want to artificially hatch them you have to keep them circulating just like the mothers do when they fan their eggs, otherwise they get fungus and are all toast.
Also, they could be immature not understanding how to hold the eggs or the males aren't mature enough and the eggs are infertile.