Indexing in Mail [Mojave]

I have Mojave on both my iMac and my MacBook Pro - this AM I tried to find something emailed from a friend [unique name] and I know the emails were still there and nothing came up. I then checked my HoudahSpot index and several emails came up. I just tried to find that same person's emails now on my MBP and it found a bunch. I did try to rebuild the folder on my iMac but Rebuild was greyed out.

Ideas? Also, is there are reason we can't just have Mail rebuild entirely or is the reindexing done via Spotlight and is there a way I can force reindexing via Spotlight without having Spotlight reindex the whole drive??