Umlaute ö,ä,ü don't show Textboxes

Hi, I have built forms that have to be filled out by users. All the textboxes for the users are set to the font Calibri. Strange thing is some of them do show the German letters ö,ä,ü correctly, some don't. I have looked through all the Settings of the textboxes, but cannot find any differences between the ones working and the ones not working for ö,ä,ü. Does anyone know where the magic button is? THANKS!