In reply to suhas.ns:
It would help to show the generate block in your RTL, but I think you are missing an instance name in your bind statement. It should be
bind top.u_dut.u_blk_gen[asrt_inst].instname my_assert u_my_assert (
If you are binding to all instances of a module, then you do not need an instance specific bind. You could do
bind targetname my_assert u_my_assert (