as title states, my left tiagra shifter on my cross bike won't 'click' into the large ring when cable attached. I tried swapping new cable on to see if that was the problem. It seems that 90% or more of the time, with no cable attached it will shift fine (there is that little orange indicator window).

Any thoughts? I guess it forced my hand on going 1x9 Maybe i'll just pick up a 39t ring at the shop after work tomorrow and do some pre-gravel group ride wrenching on the bike rack

Your front derailleur might be hitting its limit screw so early that the shifter never reaches the click. This could be due to several factors, one of which is a non-compatible FD. Pics of the FD from the rear angle might help, if your problem persists.

As stated, check your limit stop screw but that shouldn't change. As you shift, look to see what the problem is, if the cable is pulling the front derailleur cage over far enough. If not, you can manually push it a little more. If it doesn't move, the the stop is set too tight. Next, manually pull on the cable as you pedal on a bike rack. If it shifts then the cable is not tight enough. Next, disconnect the cable and shift, see if the cable moves completely. You could also have a frayed cable that is binding. Fraying sometimes occurs inside the shifter lever, and can occur inside the cable housing. Another, look inside the shift lever to check the cable end head is correctly positioned inside the shift lever. Then shift, you should see the cable inside the shifter lever move. The whole mechanism inside will turn or pivot. There's nothing else.

The shifter must be in its lowest position with all of the cable out of the shifter when attaching the cable to the derailleur.
There should be no slack in the cable when the chain is on the smallest chainring.
It may be necessary to add some tension to the cable.

Limit screws were all good. Any tension to the system causes the 'no click' situation. Deraileur shifts perfectly manually adding tension to the unit or cable...

I stopped by the LBS on the way to my gravel road group ride to pick up a 40t to get me through my ride... not too bad mated with 12-26... i guess it's my forced 1x9 setup haha

Have you tried "hosing out" the shifters with WD40? That may fix your shifters or may not but in any case it won't hurt anything.

Make sure the cable is securely and correctly attached in the shifter. Ran across a right/rear STI shifter that was doing something similar. The cable had detached and the head had jammed into something, which allowed the shifter to kind of work, but not shift to all the larger cogs.

The fact that tension on the cable causes the shifter to malfunction tells me something's up with the shifter; I doubt it's as simple as a stray cable head.

I'm thinking it's the old sticky grease hassle if you're lucky, or maybe a spring has let go.