Out of the 60ish expired domains I have bought recently, I had 2-3 that will not index, and seem to be penalized. I built links to them, gave them a few weeks and they just won't index, whereas other domains index in 2-3 days. I always check the domains anchor text and backlink profile. I make sure the backlinks are not spammy, and anchor text is clean. I check wayback machine to make sure they are not previously used as a PBN or in any of those dodgy niches that I must avoid. Is there any other way I can avoid these domains? or is it just unlucky sometimes that they look clean and good but are penalized?