Can Focus with Barlow in one scope but not the other

[SparWeb]

Why would adding a Barlow to the eyepiece in one telescope work, but not in another telescope?

I have two telescopes: a Lunt 102 for visual and photographic observing, and as a gift I just received a Coronado PST 40mm solar scope.
I've enjoyed my Lunt for many years, and I actually have two Barlows: a 2" 2x Celestron and a 1.25" 3x Televue.
The Barlows work great when looking at planets, and the adjustment in focus when swapping in a Barlow only a few mm of movement.
I admit I'm pushing things a bit with a 3x Barlow, but I get comfortable and wait for those moments of clear seeing.

When I was given the Coronado I started trying different eyepieces and it's great with a Celestron Luminos 10mm EP.
Then I tried inserting the 1.25" 3x Barlow before the EP and I can't get the Coronado into focus.

Why would the Coronado not be able to focus?
Are they different because the Lunt is a F/7 and the Coronado is a F/10?
Is my comparison with the focuser adjustment (3mm travel) in the Lunt not applicable?
Would my odds of success increase if I tried a 1.5x Barlow in the Coronado instead, and why?

[SparWeb]

Just some extra info for clarity: The Lunt is not a solar viewing Lunt, and it's not as complex as the current line-up of Lunt scopes are today. This one is over 10 years old and is much simpler, no longer being sold by Lunt, either. More like an Orion EON, really.

[SparWeb]

Figured it out.
I didn't notice the other day that the Barlow doesn't insert all the way into the eyepiece holder of the Coronado.
In fact the Barlow is supposed to insert very deeply into the OTA, but the Coronado's eyepiece holder only accepts an inch. The Barlow needs two inches.
Well, the little focuser in the Coronado isn't going to make up a whole inch (25mm) of travel, so I need another Barlow.

[Lady Fraktor]

If using a 10 mm with 3x barlow you are into empty magnification with the eyepiece operating at 3.3 mm
77x per inch of aperture would be to much for a 40 mm.
I found with my PST the most used eyepiece was a Vixen LV 10 mm