If your entries are not written to file, they will not be persistent
. You will be able to use them just for the session, because they are in memory. When you close Instant Text, the memory is gone. When you reopen Instant Text, your glossary entries are loaded back from file to memory, it is the file that ensures your entries are persistent.
When you experience a problem like that, verify upon adding a glossary entry that it is also present in the .glo file: you can open the file in Notepad for example and check.