Problem with functional coverage

I'm having a problem with functional coverage collection. When I run the following testcase and inspect the results using ICCR I'm getting zero functional coverage. The relevant coverage directories are being created and I'm seeing the cover group inside ICCR so I'm not sure what I'm doing wrong?

Any thoughts?

module test();

class gen;
  rand logic[7:0]    a;
  rand logic[7:0]    b;
 
  event              ev_collect_coverage;

  covergroup cg_all @ev_collect_coverage;      
    c_a:             coverpoint a;
    c_b:               coverpoint b;
    cx:                cross c_a,c_b;
  endgroup

  function new( );      
    cg_all = new;
  endfunction: new
 
  function void post_randomize();
    ->ev_collect_coverage;        
  endfunction : post_randomize  
    
endclass

gen testgen;
bit flag;

initial
begin
 
  testgen = new();
   
  for (int i=0; i<10; i++)
    begin
      $display("------------------------------------------------------------");
      $display("---- CONFIG = %d",i);
      flag = testgen.randomize();     
      $display("A=%b",testgen.a);
      $display("B=%b",testgen.b);      
    end
end
endmodule