Question about Onyx Storm (spoilers)

So I'm at the point where they fixed the ward stones in Basgiath to allow the riders to channel. I'm listening to the audiobooks and this quickly became one of my fav series but I'm confused about one thing; why can dragons only do the ward stones once? I know Andarna was the mystious 7th dragon and that's how they got the wards back up, but why couldn't they just go and do the process again with Andarna? Did I miss something?

If it's explained in the future please don't tell me, just tell me to keep going. But if it was already explained and I just missed it, I would love to know. Thank you!

Such a great series.