Assuming the metal parts are absolutely clean,,
I'd try a slow rust process instead of the quick/hot rust.
I used to do alot of quick rust blue and every so often a frame or bbl would do as you are experiencing and it didn't seem to matter which soln I tried on it.
Maybe the steel composition,,the manufacturing method, a combination of both, not getting all the contamination out of the pores of the steel,,aliens,,who knows...
Hot rust blue doesn't do well on hardened steel generally, but that shouldn't be of concern with the Martini.
If you're to continue to use a quick/hot rust soln,,
One trick to make hot rust blue 'take' better is on the first and second coats,,apply the soln with (de-greased)fine steel wool instead of a cloth or cotton swab.
I purposely heat the metal a bit hotter than necessary (propane torch)so that the soln sizzles when it hits the surface. I scrub the soln onto the metal working quickly to catch any runs so you don't get a pattern in the metal.
A glove is necessary as the soln in the steel wool swab will sizzle (boil) when applied and your fingers will be wrapped around it.
Keep at it till the metal starts to cool and the soln begins to dry off at a normal pace for the process.
I don't know if it bakes the soln into the steel a bit or perhaps bakes some crud out of it, but it seems to help on stubborn pieces.
Another old trick was to etch the surface with weak acid before bluing. Perhaps my technique is accomplishing nothing more than that and adding a layer of color at the same time as it does seem to matte the surface a bit more than a standard application does.
If you're using Herters Belgian Blue, wear a mask, gloves & some sort of eye protection. You don't want that stuff in or on you and when it hits the really hot metal you'll know it, feel it and smell it. AFAIK, it still has mercury bi-chloride in it.
Going back 40+ years, I used alot of the orig Herters Belg.Blue, many different small maker brands of quick rust soln and some not orig made for that purpose at all.
Birchwood Casey Quick Brown for MuzzleLoader Builders was an exellent bluing soln at one time.
One thing these all had in common was mercury bi-chloride.
Great for the intended purpose,,not great for the bluer person.
Birchwood Casey reformulated the bbl brown to exclude the merc as did most all the small lable products.
Herters went out of business.
When Brownells came out with the Herters Belgian Blue stuff again, I thought I'd try it thinking maybe they had mfg it w/o merc.
But a quick test swipe on some warm brass revealed merc plating so I left that alone.
BTW, using any of the merc containing quick rust bluing soln on parts that have inlays of gold, silver, brass, etc will leave them plated with a layer of mercury.,,an it's not easily removed either.
I use Mark Lee's Express Blue (#1?)for quick rust blueing now.
Laurel Mtn soln for slow rust bluing which I do most all my bluing by now.
Occasionally I still do some quick rust or a combination of both on the same part.
No merc in either.
I tried Brownells Dicropan back in the early 70's and never had good results with it. Still have that bottle on the shelf and have never seen a need to try it out again.
Others seem to like it though.
Good Luck with your project.
