On change of lookup JavaScript is not Triggering

Hi,

I have a JS on Change of lookup.

This JS is for if we select value in the first lookup, the second lookup should become editable and mandatory.

if we made null in the first lookup then the second lookup should become locked field.

if I select one value in the first lookup then the second lookup is becoming mandatory field.

if I removed that value then this lookup is becoming locked field.

but if I select the same which I have removed then the JS is not triggering and the second lookup is not becoming  editable.

can any one please help me on this.

Thanks.