I have a Paco frame of Columbus TSX. I bought a Redshift Sports ShockStop Suspension Stem and a Velo Orange 1 inch to 1 1/8 inch stem adapter.
I can't get the adapter inserted all the way into the steerer tube. There's over an inch of adapter above the headset nut.
I cleaned the inside of the tube really well and the adapter is lubricated.
I tried the adapter without the bolt and wedge; same result. I also tried two stems I had run previously; same result (I never notice before that they wouldn't insert all the way because they're shorter than the adapter and I didn't need to insert them as far).
It feels as if the adapter is hitting an obstruction within the tube. I even tapped it with a rubber mallet, to no avail.

What's the size of the frame? Often on smaller frames the butting of the steerer is the limiting factor on how far you can insert a quill.

On one of my bikes, I had a similar issue and it took me a while to figure it out. The headset nut was drilled slightly off center. Some stems were undersized enough to slip right in, but the one I wanted to use would jam before it was all the way in. Once I identified the problem, I used a Dremel to open the hole in the top nut a little so it was even with the steer tube.

Another issue could be if you have a smaller frame with a short head tube. The steer tube tapers inward at the bottom, so the stem might bottom out before it's all the way in.

A regular quill stem wouldn’t fit very far into a Columbus framed bike I have. It was the rifling in the steerer tube on the lower half. Perhaps the Columbus TSX tubing on yours has the same rifling. Taking off the front wheel for a look see will let you know.

It is a smaller frame, like 53cm (I forget exactly); I didn’t realize steerer tubes were butted! And TSX tubing has the “helicoidal reinforcements," which thicken the walls a bit, too.

The VeloOrange adapter is pretty long. Maybe I should look for a shorter one? I’ve had my stem at saddle level, but I’d like the option of getting it a bit lower.

EDIT: I just did some measuring. The VO adapter is 180mm (7") long, one of the longer ones available. Top of the headset nut to the shoulder of the adapter is 1 1/2", and the steerer comes within about 1/4" of the top of the nut. So I think a shorter adapter is the answer.

Thanks for the help, guys. I'll update when the job is finished.