Bike is a CAAD 10 with shifter gear levers and Shimano 105 derailleur.

Over time, shifting is working less efficiently. I'll click the shifter, I can see the derailleur move, but it often takes a second lever click to convince the chain onto a new sprocket. And often, that will be two sprockets over.

What should I look at first?

Go to shimano's support website and download the DM (dealers manual) for your particular model of 105 derailer. That will tell you what does what.

I'd just expect cable tension is the problem if it did gradually over time. If you've laid it over several times and/or banged up the DR, then the hanger might be slightly bent. Most I just replace over trying to bend it back. It's a cheap part usually.

The worst thing is to just start adjusting stuff if you don't know what it's going to do. If you've never worked on one, then realize that the learning curve can be expensive. So you might spend much more to gain experience that the bikes shop will charge to fix it.

Agreed, sounds like cable tension. How old are the cables?

If shifting from a larger to a smaller cog is the problem, cable friction is usually the cause. If shifting from a smaller cog to a larger one is the problem, a simple increase in cable tension can usually fix things

If your shifting has changed suddenly, or if it has been a while since you have looked, check the shift wires inside the brifter for fraying. MUCH better to catch it before it breaks off.

Excellent points, everyone!

cables are original, from 2014. I probably ride about 30 miles a week?

It seems to be both directions - small sprocket to large, large to small. And yes, it's been gradual.

The consensus seems to be start with cable inspection and stay away from adjusting derailleur.

"Brifter" = break integrated shifter?

Thanks!

With the limited information available I'd guess (90+ % sure) you need new shift cable housing. Specifically that little piece that loops behind the derailleur. A little too long for that little loop is better than a little too short.

Maybe a bit of lube as well?

Sounds like a minor enough job for me not to screw up. And if I do, I can limp the thing to the bike shop.

Preciate!

OMG 2014 cables???
Even with low usage, that's pretty old. No matter what you should change the cables and housings. You'll notice a huge difference!

Cables can fray in the shifter after a while. Don't let it get to that point.