That is because they haven't offered anything to Putin. What they call "diplomacy" has essentially been "Putin can you pleaase be nice and pull out?"

Appealing to Putins empathy isn't going to get you anywhere. You got to put real stuff on the table.

Giving Crimea and Donbas to Russia has been off the table. That is an absolute minimum to get Russia to pull out and they haven't even offered that.

Yes, I know it is totally unreasonable. That I say this must be on the table doesn't mean I think Putin and Russia is in ANY way entiled to have these territories. This is simply realpolitics. It is about realizing what must be sacrificed in this difficult situation.

A compromomise could be that a vote is carried out in both regions which is observed by both parties.