If you have treated your kitchen twice and the roaches returned both times, the treatment was probably fine. The problem is the building, not the apartment.
How they move between units
German cockroaches live in warm voids, and a pre-war building offers plenty: plumbing risers, the chases behind kitchen cabinets, motor housings on refrigerators and dishwashers, and the gaps around waste stacks. These run vertically through the building, connecting your kitchen to the ones above and below.
Why single unit treatment fails
You reduce the population in your unit. The neighbouring population, untouched, migrates back along the same route within weeks. This is not a failure of the product, it is a failure of scope.
What actually works
Treating a vertical line of units at the same time, using gel bait placed into harbourage rather than sprayed on surfaces, combined with a growth regulator to break the breeding cycle, and sealing the chases where practical. Boards that fund line treatment once usually stop having the conversation altogether.
