Cannondale brifter stuck?
 

My wife just got a used Cannondale Synapse. It it almost new but sat for a few years.
The brifter doesn’t want to move to downshift. It did upshift.
I won’t force it, but it feels like it is hitting something when I try to move it.

What brand/Model is it ?
Iinformation is always a good thing... pic of physical condition, etc...

Left or right shifter?

Shifter. It's a shifter. We know it includes a brake lever. What brand/model? You haven't given any useful info at all. And when you say 'upshift' you do mean shifting to smaller cogs, right?
Also this is a mechanical issue/question so it shouldn't be in GD, it should be in 'bicycle mechanics'.

I had a stuck one when I shifted to the smallest rear cog once. Cable end popped out inside the shifter and jammed in the mech. I would check under the rubber hood. Once I got it popped back in and adjusted the rear DR it was fine.It could be hung up in the housing etc too. If the cable moves freely in the housing thats probably good.

I don't care for the term brifter but there is a Sub forum on BF with the term so I guess its legit. Lol

They're gummed up. If it's the older style with exposed shifter cables then squirt some wd-40 in them.

The Cannondale I bought in early spring had inconsistent shifting RSX brifters. They responded really well to the WD40 flush (make sure you remove the hoods first). Google it- it's pretty simple, but there are some posts/videos etc about doing it. Can't remember now, but I think I used some light lithium grease after flushing/drying.

It could be as simple as a bad shifter cable that needs to be replaced. I would start by disconnecting the cable and seeing if you can get it to downshift and upshift as you put vary tension on it.

Right.

Shimano Flightdeck shifters, Ultegra groupset. New condition.
When I push on it I can see the cable housing flexing.
Can the derailleur be sticking?

That doesn't tell us much on the model/speed.
But if cable housing is moving when you try to actuate, then release/unscrew the cable from the deraiileur (FD or RD ?) pull cable housing a few inches off shifter to expose the cable there, grab cable and see of it slides smoothly - will tell you ff problem lies at shifter or at some point AFTER the shifter - might be an 'partial torn'/unraveled cable stuck at a junction or in cable housing... a rusted/unraveling cable is mostly likely... replace cable and likely best to replace the cable housings as well.
EDIT: ... like Veloz said...

I am not sure how many speeds it is. I will count the rear cassette later.
It is a Cannondale Synapse, all carbon with Ultegra groupset. It has rim brakes, not disc. Is there a way to figure out what year it is by the serial number? I really haven’t taken the time to look for decals etc.

You're asking questions that can't really be answered here.

If the forum member wants to call it a brifter, it’s a brifter. You are not the person who determines what someone should call their components. Stop it.

If you pull back the hood the shifter will be stamped with a ST-XXXX that will identify the model. If flightdeck though I'm betting 9 speed.

Certainly not with that attitude.

I was just hoping it was a common issue. If I can’t figure it out I will take it to my local bike shop. It wouldn’t hurt to get it checked over anyway.

I just thought that was what they were called.

You’re fine. It is what they are called.

oh, it doesn't matter at the moment, which model... check the cable and housing... likely where the problem source is.

It is what they are called. There is a sub forum here with the term in the title. I had never heard it until I came to BF. If ur not cool with wrenching on the bike I'm sure this would be easy for your LBS to handle.

no problem. You should watch some of us old scuba divers get upset when someone calls them oxygen tanks.

You're correct. They are brifters. Brake/shifters.

New condition flightdeck?...that's probably 15 years old or more. Ultegra 9sp, I'm guessing.
Odd that they would be new condition.

Undo the cable pinch bolt at the rear derailleur so the cable is slack. See if the shifter then clicks without any tension. That is always my 1st step to determining what's wrong.

If it clicks as it should, then you know the shifter is fine and the issue is elsewhere(cables, housing, rear derailleur). If the shifter doesn't click as it should, you know the issue lies with it.