Never had that problem on the 'puma' engines as fitted to Mondeo/Transit, and I've fitted a lot of injectors to them over the years.

The little 1400/1600 DW French diesels as fitted to Fiesta/Fusion/MINI and all sorts of French shit have a tool that's like a T handled wire brush that goes into the injector hole to clean it out when they have suffered blow-by.