Odd merge behaviour involving reverts

I am encountering an odd problem with how merging branches are working.
I expect I am doing something wrong, I just need to know what.

Here is my situation...

I have a master branch.  We have a branch off of that that some
developers are doing work on.  They claim it is ready. We merge it into
the master branch.  It breaks something so we revert the merge.  They
make changes to the code.  they get it to a point where they say it is
ok and we merge again.

When examined, we find that code changes made before the revert are not
in the master branch, but code changes after are in the master branch.

We now have code that does not contain all of the changes in the branch.

Is there a different way to undo merges into a branch that does not
block further merges from that branch?

What am i doing wrong here?