problem in Item Event from base document

Hi

I want to add message box showing "DocEntry Number" , after posting AR Invoice or AR Credit Memo

When I post it directly posting (without base document), then message box  shows

but when tried from sales Order => copy To Invoice and post  that message box not appear,

I also notice, if I create draft invice from sales order and then post, draft invoice, message box appear

try to trace but, cursor not came after posting invoice from Sales order

Let me know if any help

Thanks

Nilesh Shah