Scenario 1)

If you are trying to merge 2 tickets  and tick one and then scroll down the toolbar disappears sometimes in Chrome as demonstrated in the above chat session.  I have attached a screenshot to explain.

Scenario 2)

You are trying to merge 2 tickets where the second ticket falls on another page (>50 tickets)  you cannot merge across pages.

I know you can do an advanced search, but that is really clunky and time consuming and defeats the object.