It can be hard to tell whether worming treatments are working. If a flea treatment doesn't work, you end up with a cat scratching like mad but it's not always so obvious when a cat has worms.
Having said that though, Dronspot seems to do the job for my lot. To the best my knowledge, it's the only 'spot on' worming treatment that kills all types of worms (unlike similar worming treatments sold in supermarkets, which only kill certain types) which it's possible to get hold of if you've not taken your cat to a vet's surgery in the past 12 months. (So I don't have to go through all the hassle of trying to catch my cats and get them into a pet carrier!).
It can be ordered online from Pets at Home:
https://www.petsathome.com/shop/en/pets/dronspot#
(You can't get it in their stores because orders have to be approved by a vet, which involves a bit of form-filling for the order but it's very straightforward).