Some parts of model are not transparent


I recently finished modifying certain models (.nif meshes from Skyrim to be precise) and wanted to add alpha transparency. I had done that before with original models and there had been no problem, but altered ones have some faces that are not transparent. I tried to recalculate normals but it didn’t help. I’m not a profesional when it comes to using Blender, I just know basic things which I’ve learnt by myself. Here is a picture showing the problem:

Does it look correct in a render or in Rendered view? The material preview will look like this with translucent objects despite actually being correct and looking fine when rendered, no idea why and as far as I know and have found there’s nothing you can do about it.

In blender it’s almost not visible, you can see it only in this spot:

In-game it looks similar, but if I set “double sided” shader, it looks like on the first picture.