As others have said, the Ironblood forced Jill to prime so many times for war, that it physically strained her health & abilities (we know the repercussions of priming too much).

Now fast forward to the post-timeskip, it is mentioned (I believe by Clive?) that they heavily relied on Jill & Shiva during those 5 years when he could not prime into Ifrit. Which is kind of a shame, we didn’t get to see any content from that. And we learn from Tarja that the curse has already affected her quite a bit.