Merge Asset Issue

Hi Guru's

I have a Confusion regarding Merging of Assets.

I want Merge 2 Assets. Merging of Assets can be done before posting the Assets I believe. Correct me If am wrong.

Path: Mass Additions-Prepare Mass additions

When I select a Invoice whose Status is "New" & I click on Merge Button, Merge window opens & there am unable to merge the Invoice as the check box merge for some Invoices But for Other Invoices whose Status is New, when I click on Merge Button, there I have the option to select Merge check Box.

2.  Now My doubt is :-

a. Why the merge Option is available for some Invoices & not available for other Invoices where the  Status is New for both Invoices.