We were roach free for 15 years then on a visit 4 years ago we had several running around, like you house is scrupulously clean. It took 2 years of visits before we got on top of it. On leaving each time we sprayed and put down 12 traps on arrival back months later usually 1 or 2 dead but none live. We then started sealing each room, by closing door and stuffing material under door, the entry points were the fan grill in bathroom which vented onto room chimney so meshed and sealed, the next was the extractor hood in kitchen again this led to another chimney vent on roof, expanding foam and mesh sealed that. The other culprit was furniture which came in from outside at the end of the season, any eggs insects lying dormant got taken in like a Trojan horse.