Datatypes not staying fiwed

I am having issues with tags created using a datatype reverting to an older version.
I have several identical pieces of equipment that I created a datatype for.
As I progress in the project I add more items to the datatype.
On at least (2) occasions I have had the the machine tags revert to an ancient ( many revs ago) datatype.
I have to reassign the datatype to force the tag group to act correctly.
What am I doing wrong?