Which ones?
Indiana is going through a very divisive primary. The Republicans have all been running ads tearing down their opponents and regardless who the winner is you have a divided base and loads of ammunition given to the Democrat. Admittedly this is the one I think most likely to flip but it was GOP screw-ups six years ago that lost it in the first place. Don't be so sure that they've learned since then.
In Missouri Hawley is the current AG and is involved with prosecuting the sitting Republican governor. There is no way he comes out looking good on that. If Greitens is impeached then Hawley has to handle that. If Greitens resigns, unlikely but possible, then his supporters aren't all that happy with Hawley either. Add to that the fact that Hawley has run for only one election in his life, the AG race. He may not be ready for prime time. He's already made some minor verbal gaffs. A self destruction ala Todd Akin is not out of the realm of possibilities. He can win but he needs to run a flawless campaign. Can he?
In Montana Tester has already won twice in a state that has gone for a Democrat president once since 1964 - and that was because Perot split the vote. He's known, people like him, and his opponents are not really big names in the state regardless of who wins. Tester will likely win a very close race, but he'll win it.
In Ohio and Michigan neither Brown or Stabinow has a strong candidate to run against them.