Let us first talk about why this could occur in the first place:
A. Submodule(s) have been edited in place.
B. Submodule(s) committed, which updates the hash of the submodule that is being pointed to.
C. Submodule(s) not pushed.find exact fixing commit or version.
Solution for this problem would to perform push after modifying the submodule
$ cd submodule
$ git push