In reply to nishita_tailor:
You didn't show how clk was generated to sample the covergroup, but assuming it did 8 samples, I don't see how you would have gotten more than 50%. coverpoint_x/y have 4 bins each that are all hit (8 bins). But cross_xy has 16 bins of which you only hit 4. So there are only 12 bins hit out of a total of 24 bins which is 50% coverage.
To get 100% coverage you need all 16 bins of cross_xy hit. You would need
bit [1:0]a[16]='{0,1,2,3,0,1,2,3,0,1,2,3,0,1,2,3};
bit [1:0]b[16]='{0,1,2,3,1,2,3,0,2,3,0,1,3,0,1,2};
