Help with Blender, please

[MOLOTOVANZA]

Hi, I want to ask you for some practical advice on modding regarding Blender.
I'm not new to modding but I'm certainly a beginner compared to you, so I need your advice in form, clear, simple and expert.

I'm creating a mod to change Arcade's outfit, and also his aesthetic.
He will be dressed in 19th century style, with a monocle over his eye, a top hat, an elegant suit and a doctor's case ALWAYS in his left hand, so with him I will always adopt the use of one-handed weapons.

The specific problem here is that I don't understand why on Blender I create a model, actually, I merge two .nif files into the same collection and then on the GECK I don't see that model, but when I insert the .nif file this object is visually not seen, it is invisible .

The doctor's bag is the one that gave me problems.
I took the file of a left hand [lefthand.nif], from the \Data\Meshes\characters\_male folder, I took the nif file of a doctor's bag already present in the game [doctorbag.nif] from the \Data\Meshes folder \clutter\health, I worked these files together, spent about 3 and a half hours in total.
-I straightened the handle of the briefcase vertically, so that it appears to be held in one hand
-I bent the fingers of the hand of that .nif file so that they held the handle of the briefcase
-I inserted the briefcase inside the hand and finally I also reduced the size of the briefcase to avoid clipping with the new Arcade clothes.
-Finally, and perhaps I made a mistake here, I deleted the collision file linked to the briefcase on NifSkope, which was still in its original position and I had not repositioned it on the briefcase.

I saved a .blend file, I finally exported the work in the form of .nif and when I open it with NifSkope everything is perfect, both hand and briefcase can be seen.
However, when I create a glove on GECK and set the model to this .nif I don't see anything, everything is invisible.

What do you think I did wrong?
If you need it, I am available to share these work files to show you and help me.

Thank you very much to everyone who will help me, they will be mentioned in the mod when I publish it on Nexus.