Textures not changing on some computers

We (a group of 4) are trying to change the grass block side texture in the test client.  We created the minecraft folder in assets, put a textures folder in it, a blocks folder in that, and then our respective textures in the blocks folder.  We named the texture grass_block_side.  On one of the computers we're using, it works fine; on the other three, nothing changes.  The folder structure is identical on all four computers.  What could be the problem here?  Is there a step we could have missed on some of the computers?  As far as I know, after doing what I described above, it should just work.  Is there another step somewhere?