in general, it would be hard to intelligently say what is wrong as someone would need sample data or at the least a picture of your DM to understand what is going on. however, as a guess, i think it would fail here:
ONLY({name={'a','b'}>} name)
only will come back with a value if there is only one value for name however, you told it to pull 2 values a and b so it would come back with NULL
