[Koha] Cannot find records created in KoHa

Hi, I guess this is a very basic and silly question. I just created my first record with some items. Among other things I filled in the field "Number (ISBN)” with 100.100. Using the advanced catalog search I’m trying to find my record, but nothing is found: No results match your search for 'nb: 100.100'. What did I miss? Best, Patric