It may have nothing to do with your problem but there were houses near here with a similar problem some years ago. The downstairs was open plan and there was meant to be a 12" x 12" main joist across the house that the other floor joists rested on. The builder had fitted a 10" x 10" (or worse) and the upstairs floor had sagged, leaving inch gaps under the stud walls in their centres upstairs. This was eventually sorted under the NHBC guarantee (as they were not built to plan) AFAICR.
Good uck, after 16 months you must be a bit upset (in spite of our clever ways on this ng, most of us would be too!).