If it is the case that DLM thinks your line is 'too stable' to need g.inp, then that makes no logical sense, in terms of it being a requirement to have g.inp applied before you can get xdB.
If your line is that stable, then surely you should be able to have xdB without g.inp, or be given g.inp *automatically*, as a means to getting xdB?
I'm more inclined to believe that the people with 'stuck' lines that don't get g.inp are on cabinets that have the DLM reporting issues that were discussed many months ago. If that issue does still exist, and it only takes a single modem on a cab containing hundreds of lines to 'break' DLM for the whole cabinet, then I'm pretty sure that many cabinets are affected.