Make sure that wherever you're getting it primce-matched to - be that B&N, Kobo or wherever - the title/subtitle/cover, everything are exactly the same, same formatting, same everything.
One small difference in any of those is enough to trigger the bots to stop price-matching it. I had this issue and it was a nightmare.
Pricematch>un-pricematch>pricematch>un-pricematch.
It wasn't until a sharp KDP rep noticed that the cover and the formatting of the title were slightly different on D2D that it got sorted. Once I corrected those, it's stayed permafree since.