I think you may have the same problem I had:
I wanted to change a named texture on a model, as I was editing it, to see the differences.
It seemed as if the graphics card was saying 'No, I've already got that (named) texture in memory, I won't actually reload it.' - but no error in Blitz.
The only answer was:
a) To load a texture of a different name (obviously), or
b) Erase the model, and reload using the newly changed (but same name texture).
I wrote my own model viewer, and had to end up using method b) - it was the only way to be sure.
I wanted to change a named texture on a model, as I was editing it, to see the differences.
It seemed as if the graphics card was saying 'No, I've already got that (named) texture in memory, I won't actually reload it.' - but no error in Blitz.
The only answer was:
a) To load a texture of a different name (obviously), or
b) Erase the model, and reload using the newly changed (but same name texture).
I wrote my own model viewer, and had to end up using method b) - it was the only way to be sure.