Sadly, some applications can be tough to package. Sometimes they need a patch to work with the non-standard file system hierarchy enforced by the store. Sometimes the tests won’t run properly. (They can be skipped but this is not recommended.) Other times the resulting package won’t be reproducible.
Should you be stuck, unable to figure out how to fix any sort of packaging issue, don’t hesitate to ask the community for help.
See the Guix homepage for information on the mailing lists, IRC, etc.