Sorry the eggs are gone. You'll get more.
I've had eggs on filter tubes, heaters,
driftwood, right on the tank glass, and mostly sword leaves, but I don't think any of my pairs have ever spawned on the slate. They don't do anything I tell them to.
My nine year old son is doing geology in his third grade class and is having way more fun with the slate than the fish ever did. He writes on it with another rock like a chalkboard. Oh well.
Your angels should lay more eggs in about 2 weeks and it's true they do get better at it each time. Even if they're the only ones in a tank they may just eat the eggs sometimes.