While Merging-, Assets do not Appear in Merge Window

Hi,

While performing merge of Assets for one particular AP Invoice  some lines do not appear in Merge window, although these lines are appearing in Prepare Mass Additions with Status of New.

Some lines are already in Merge Status and grayed out for this Invoice, but few lines with status New do not appear while Merge?

What is the reason for such Invoice Lines not appearing in Merge window and how to check this issue?

Rgds

Raju