Sometimes they work. Sometimes they disappear.
That suggests that they don't actually have global storage. Either that or something else is affecting their visibility such as a conditional format or something is clearing the value entered...
Sometimes they appear but the value can't be changed,
This is a known bug in FileMaker 10 and 11 that does not occur in FileMaker 12. Try making sure that the current record is the first record and your window is scrolled to the top of the page and you then should be able to edit the values in these fields.
For More Information see: Sometimes global field in header become disabled
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Thanks Phil. Even with a script trigger to "go to first record" the global field in the header remains unreliable. Time to upgrade to V12. I appreciate your clarification.