Appearance of damp after dpc injection - can "good enough" plaster go soft?
"Deficient" render? That is a line of bull. Your use of damp proof course and render makes me think you are UK based. If so, you should have a guarantee of several years for the damp course work, and if so you should demand they fix the problem as they clearly missed a spot. Residual moisture? Don't make me laugh. They need to do fresh moisture measurements and to re-course the areas that still show high moisture content. 

At the same time look at your exterior brickwork in the area of the damp and make sure that your pointing and exterior render isn't blown. Also, when it rains make sure your guttering is piping the rain down to the ground properly.