I wonder how long you've been playing. The words you mentioned are not at all obscure. There are only just over 2,000 in the database. I can agree with you on those few where you can have 5 out of 6 letters in the correct spots, but still have 4-5 choices, but that has only happened to me once. Up your game!
I can see "tacit" as it's not well known for those who aren't big readers, although I think it's a great word that should be used more... "tacit approval" is the most common phrasing these days. "Swill" and "caulk" I would think are not so rare. I didn't know people don't swill beer anymore, that would be a good thing, except I believe they chug it now instead. But there's no substitute for caulk, you can't live without it, if you ever must fix a window seal or a leak around a sink, tub, or toilet you will become intimate with caulk.
I see what you mean on the "shake" one, where any number of letters can be substituted for one particular letter. I think I pulled that one out at the last step!