Damp proofing done but plaster not completely dry after 3 week

By now the plaster should have dried, maybe there was and issue with the installation of the damp proof system or potentially you've got water coming in for somewhere else.

They may be several answers : Damp proofing was’t done correctly , and the damp still going through . The wall was’t dry completely before damp proofing . The best way to solve these types of problems is to inject a special liquid into the wall from damp . Paint the brick wall with professional anti mould-damp liquide , and let dry it . Build the stud wall next to the brick wall keep around 20 mm air gap between . The damp doesn’t go through .

Depending on what plaster method was used If they used plaster board and skimmed it should be dry If used sand cement render will take longer If you live in house with heating then after one month their is still an issue

Hii 3 weeks is quite a long to dry but you can try dehumidifier maybe on that room is not hot enough and after repaint all that wall Thank you