problem with alpha channel

I have some triangles made with the opengl way (glBegin(GL_TRIANGLE_STRIP);…) in 3d space but their alpha channel is only visible when viewing them from the right. When my camera is on the left of them they are being presented as if they have no alpha channel, only r,g,b.

It happens whether i use glColor4f or ofSetColor.(i dont know their difference anyway)

thank u very much!