How long does it take for those flea ampoules to start being effective on dogs?

I am a very careful dog owner and take care of my beloved dog really well, especially when it comes to hygiene. Lately she has been scratching like crazy! No baths have done anything for it. I noticed what seemed like two fleas so I applied an ampoule and got a flea collar. Three days later she continues scratching. I am confused. Could it be an allergy? How long does the flea medication take to work? Should I empty another one on her?
I hope she won’t need pills!