"at the top" suggests that these are in the header. Sounds like a known issue with global fields in the header.
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 here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
Yep, that does look like the issue! Too bad that they aren't forthcoming with information about whether or not they plan to address the problem, or when. I was surprised to see that it has been around since FM 10! This seems like an important issue.
(Just explaining, not defending....)
FileMaker Inc. makes it a policy to never announce what new features or bug fixes will be included in the next release or bug fix update nor when they will release it. Some even more significant issues are much older than this one...