Most people treat cockroaches the same way: see one, spray it, see fewer for a week, then find them back again. After a few rounds of this the conclusion is usually that the spray is not strong enough. It almost always is. The problem is that spraying answers the wrong question.
The cockroaches you see in the open are a small minority of the population, and they are usually the ones under most pressure — crowded out of the harbourage, or foraging further than they would like. The bulk of an infestation sits in the dark, in wall voids, behind and inside appliance housings, under sink units, in the cavity around pipework. That is where they breed, and nothing you spray on a worktop reaches it.
There is a second problem. Many retail sprays are repellent, which sounds useful and is not. A repellent product makes treated surfaces unattractive, so cockroaches avoid them — and in avoiding them they disperse. A population that was concentrated behind one cupboard spreads to three, and the infestation becomes harder to clear than it was before you started.
This is why professional cockroach work is built around gel bait rather than spray, particularly in kitchens. Bait is not repellent; the cockroaches are drawn to it. They feed, return to the harbourage, and the active ingredient passes through the aggregation by contact and by the routes cockroaches share food. It reaches the insects you cannot see, which is the entire point.
It also means the visible result is slower. A sprayed kitchen looks fixed the same evening and is re-infested within weeks. A baited kitchen shows activity for several days and then falls away properly. If you are paying for cockroach control, the slower result is the better one.
Two things you can do that make more difference than any product. First, deny them water — cockroaches will find food anywhere, but a dripping tap, a wet mop left in a bucket or a saucer under a fridge is what actually sustains a population. Second, do not clean or spray near bait placements for a fortnight after a treatment. Strong cleaners and insecticide near the gel make it unattractive, and cockroaches will walk past it.
It is also worth knowing which species you have, because it changes the plan. German cockroaches are small and tan, live in kitchens, cluster around appliance motors and breed very fast — these are the ones that need patient baiting. American cockroaches are much larger, usually arrive from drains and sewers, and are more about drain hygiene and sealing entry points than about baiting at all.
If you have sprayed repeatedly and the problem keeps returning, stop spraying before you call anyone. A recently sprayed kitchen is harder to bait, because the residue makes the placements unattractive and pushes the treatment out by a week or more.