Eggs not Sticking

The Angelfish again.

They regularly spawn, even at a temp. of 25*C. The only suitable spawning place in their territory is an Anubia, which has quite some algae on it. It's green and quite hard algae, nothing fluffy or fuzzy like BBA or Staghorn.
The Angels pick at the leaf to try to clean it, but the algae stays. When the female try to lay the eggs, the eggs keep falling off the leaf. They don't get anywhere.

What could the cause be? Is it the algae, or is it the diet or water conditions? I feed Omega One flakes and some of Prof's frozen food every now and then when I have time. This particular pair have spawned before without this problem.

Thank you.