Often opening files in Libreoffice doesn't show the window until you click on the taskbar. For example, open a .csv file via libreoffice while Calc is already open - it won't change focus to the instance of libreoffice which is trying to open it. But Impress also does this under certain situations.
Is this on Windows, Linux, macOS..? I know there is an existing report for Windows. If you have Windows, you can try searching for it and close your report as duplicate.
It's on windows, but I can't find an existing bug report for this issue-
*** This bug has been marked as a duplicate of bug 96011 ***