Do you have any idea where the rats are coming from? Is there a particular source that you suspect eg a broken drain, or somebody else's property. Maybe you are not sure? Am I right in thinking that you live in a rural area? What are your site characteristics and history? What is the extent of the infestation? Is there a history of bait shyness?Where are the harbourage locations? Have you tried clearing the area of any debris/vegetation to create a clear perimeter(rats don 't like the open)
Professionals don't always use the strongest poisons;they act quickly and if the rat survives an initial nibble it will learn to avoid that product. If you were in a resteraunt and immediately became ill you would blame the food. However if you didn't get ill for a few days which of your meals would you blame. Sometimes we want a slow acting poison so that the rat carries on eating the poison and eats enough to kill it.
Different products are used in different circumstances.I must be careful about advising particular products because you can have the 'best product' in the world which will kill a lot of rats but it won't get rid of the infestation . It's also about proofing(difficult to do outside), removing food/water sources and harbourage, housekeeping and lots of other things-as I am sure you must be aware ,as you have had some success..but I would say I have seen amateurs have success with stuff bought from B and Q which is basically the same stuff professionals use but at a lower concentration.
Look at the active ingredient on the packaging ,what does it say? Here are some active ingredients:
Difenanacoum(seen this in various shop products) or bromadiolone are 2nd generation acute rodenticides that were inroduced to overcome resistance from warfarin.
Brodifacoum(seen this in 'amateur' products in shops)-highest toxicity, also a 2nd generation rodenticide.
there are others but only available to professionals.
How and where have you been baiting? in the open, in a rat box? have you been using pasta type bait,grain,blocks? I would be curious/nosey as to what measures you have taken. If you need a professional use the BPCA website to find one. Here is a link to some advice on DIY pest control..
https://bpca.org.uk/a-z-of-pest-adv...-of-and-prevent-rats-bpca-a-z-of-pests/188991
happy to waffle on more if you have any more questions-keep em coming( not the rats though)