see background through two translucent textures

[bjbaidu]

I draw 3D cloud, it is a dimensional rectangle(x,y,z > 0) textured with a RGBA cloud bmp image.
if the camera is some where at the assurgent, some translucent cloud looked have opaque caps, As if the translucent front side of the cloud together with the translucent back side of the cloud result in opaque, so I can't see background color through the opaque caps.
I suppose the translucent front side of the cloud together with the translucent back side of the cloud will also result in translucency.
here is my code:
//begin
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glClearDepth(1.0f);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LEQUAL);
glEnable(GL_TEXTURE_2D);
glEnable(GL_LINE_SMOOTH);
glEnable(GL_ALPHA_TEST);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glTexEnvf(GL_TEXTURE_2D, GL_TEXTURE_ENV_MODE, GL_REPLACE);
glHint(GL_PERSPECTIVE_CORRECTION_Hint, GL_NICEST);
glShadeModel(GL_SMOOTH);
glMatrixMode(openGL.GL_MODELVIEW);
glLoadIdentity();
glPushMatrix();
glLoadIdentity();
glTranslatef(fMoveX, fMoveY, -2.5f);
glRotatef(fAngleX, 1.0f, 0.0f, 0.0f);
glRotatef(fAngleY, 0.0f, 1.0f, 0.0f);
glRotatef(fAngleZ, 0.0f, 0.0f, 1.0f);
glScalef(Scale, Scale, Scale);
if (Width1 > 0 && Height1 > 0)
{
glBindTexture(GL_TEXTURE_2D, intexture[1]);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameteri(openGL.GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,GL_CLAMP);
glTexParameteri(openGL.GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
gluBuild2DMipmaps(GL_TEXTURE_2D, 4, Width1, Height1, GL_BGRA_EXT, GL_UNSIGNED_byte, m_symbol);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
float[] light_position ={ 0.0f, 6.0f, 3.5f, 0.0f };
float[] light_ambient ={ 1.0f, 1.0f, 1.0f, 1.0f };
float[] mat_env ={ 0.75f, 0.75f, 0.75f, 0.0f };
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);
glMaterialfv(GL_FRONT, GL_AMBIENT, mat_env);
draw(); //glDrawElements()
glDisable(GL_LIGHTING);
}
glDisable(GL_TEXTURE_2D);
//end
please tell me where is the mistake

[ZbuffeR]

glEnable(GL_DEPTH_TEST);
Depth testing only works properly for totally opaque objects.

If you first draw front face of cube, then back face, the depth test will skip back face totally, and you will only see front face.
If you first draw back face of cube, then front face, the depth test will do nothing (all pixels pass) and you will see both back and front.

A lazy fix is to just disabled depth test for translucent objects. The correct fix is to manually sort faces at each frame, and draw back to front, because GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA is also order dependent.