Roundworms are very easy to pick up from the ground when they are scratching around and pecking. The eggs survive a very long time in the environment, and reinfection is common. Sometimes after worming with safeguard you will see dead or dying worms expelled. The only way safeguard is effective in feed is explained here:
https://www.backyardchickens.com/threads/safeguard-mash-zero-day-egg-withdrawal.1254653/
This method is usually used if someone wants to avoid egg withdrawl. Personally I think it's best to dose each bird individually, to make sure that they get the correct dose. Oral syringes are available at many feed stores, tractor supply, and most pharmacy's if you ask. If you have never given medication orally, this is a good article:
https://www.backyardchickens.com/ar...dications-to-all-poultry-and-waterfowl.73335/Dose for Safeguard liquid goat wormer is .23ml per pound of bird weight (a digital kitchen scale works well). For roundworms you can dose once and then repeat in 10 days, or if you want to cover the bases for most common worms, you can dose 5 days in a row. Not knowing how you dosed, I can't say if they got enough or not, but I would be sceptical. Under dosing can breed parasites that are resistant to the medications.
DE is ineffective for internal parasites, I wouldn't use it. The eggs in the environment are very hard to get rid of. Keep droppings cleaned up as well as possible, keep feeders and waterers clean, and treat as necessary. With roundworms I treat the whole flock, since if one has them then they all have been exposed and it's not really cost effective to get a fecal on each individual bird.