Cannot change table width

I have a table (imported from Word) which will not let me change its width. I can change the width of the columns within it - and I can add columns - but cannot change the width of the table itself. Other tables in the same document do not have this problem.

If I use the metrics or table inspectors I can change the size entries - but it just shrinks again when I leave the entry field.

I cannot find any setting that would "lock" it like this - can anyone tell me what I am missing.