OpenGL ES iOS gl_PointSize doesnt work

I have to draw many Points an the get drawn as well but only one point is big(I see the other points but they are very small).

In my vertex shader i said :

gl_PointSize = 16.0;

My Glkview:


- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect{

glClearColor(0.65f, 0.65f, 0.65f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glBindVertexArrayOES(_vertexArray);

// Render the object with GLKit
[self.effect prepareToDraw];

glDrawArrays(GL_POINTS, 0, 36);



// Render the object again with ES2
glUseProgram(_program);

glUniformMatrix4fv(uniforms[UNIFORM_MODELVIEWPROJECTION_MATRIX], 1, 0, _modelViewProjectionMatrix.m);
glUniformMatrix3fv(uniforms[UNIFORM_NORMAL_MATRIX], 1, 0, _normalMatrix.m);

glDrawArrays(GL_POINTS, 0, 36);

Thanks in advance