zoom button Not Working Is Greyed Out In some custom form

Hi

we developed some custom form in R11i and upgraded the system from R11i to R12.1.3 recently.

After upgrade,when using CUSTOM.pll to enable the ZOOM button,

we found zoom button Is Greyed Out on some custom form not all.
could you please share me why the issue occur.

Below is the code we add to CUSTOM.pll CUSTOM package body
----------------------------zoom_available----------------------------------
  function zoom_available return boolean is
  --
  -- Real code starts here
  --
  begin
    return TRUE;
  end zoom_available;

-------------------------------event---------------------------------------
  procedure event(event_name varchar2) is
  --
  -- Real code starts here
  --
  l_function_name varchar2(240) := fnd_function.current_form_function;