fleas have become resistant to some poisons, so if they've had drops but still have fleas you should try another that has a different ingredient. You should wait 4 weeks before putting any more drops on them though because poison.
Yes. And you said they still had fleas. So work out whatever the vet gave them, look up the active ingredient then use a different one, as long as you leave 4 weeks between applications.
fleas have become resistant to some poisons, so if they've had drops but still have fleas you should try another that has a different ingredient. You should wait 4 weeks before putting any more drops on them though because poison.