merging two commits skips a file from the merged commit, using git checkout -- filename brings the changes

Question

i have two commits A, B
i want to merge B into A

git checkout A
git merge B
this skips a file(does not merge it even though the file was changed)
if i discard the merge and
checkout commit A (detached head)
git checkout B -- fileName.txt (this changes only the selected file to the B version)

then the changes are brought correctly.

to double check i used
git diff A B -- fileName.txt

And it showed that there are indeed changes between the two commits regarding this file.
Is there a way to make sure that the merge didn't discard other files like it discarded this file ?