That could be the problem, what it your GH and KH (I think you have said before but I have forgotten). Some people have good spawns in hard water like zman said, but I think the general concensus has better luck with soft water.
I have a pair that just eats the eggs every time, I know they are male and female and my water is very soft, but I am ready to give up on them. My pH stays around 6.5 and GH and KH are 4 dh. I have had other pairs spawn, I have another pair that spawns regularly, but this pair is a dud. I have had them in the same tank for about one year now so I think it is time to give up! Yours have not been together for very long so it may be they are immature, could be water. Only time will tell.
Got to love them fish, they are so unpredictable!