Fleas even with Frontline Plus

It had only been 10 days since using Frontline on Hoshi and I noticed him scratching. I also noticed flea dirt and fleas on his scrotum during his trim yesterday so I used Hartz UltraGuard Plus flea shampoo on him. I’m not sure if the Frontline didn’t work or if he picked up more fleas outside. He’s seems comfortable now.