Could anybody help me?
If you haven't already found a solution to this issue, hopefully these points of consideration will help identify the solution.
1) Use textures and not renderbuffers
2) glGenerateMipmapEXT will generate the mipmaps for the specificed texture target (GL_TEXTURE_2D) of the active texture - so make sure you have the desired texture unit active.
2) The max level for the texture should be set at the highest mipmap level that you'd like to generate and that the base level is 0.
3) You may also need to make glTexImage2D(..) calls with the same components and formats for each of the levels that you'd like to have generated.
Use the TexLevelParameter functions to get information on the levels and look for any errors that could be getting returned which may guide you.