Steps to reproduce:

1. Create a data object with a few fields
2. Create a Guided decision table with at least a condition column binding that object
3. Go back to the object definition and add a new field and save
4. Go back to the decision table and add a second column condition on the same object: the newly added field is not available among the other old fields in the drop down menu

Note that if you create a brand new table and try to add a condition on that object than also the new field is available.