Just spend as much time as possible with them and they'll warm up to you. They're going to be more nervous as ducklings because everything is a threat. As they get older they'll calm down.
Where do you keep them? If possible, have their brooder in a place indoors that gets a lot of traffic, so they can get used to people walking around them.
At first it might be helpful if you have 'sessions' where you lay on the floor with them and let them come to you. When they come to you, don't reach out, don't make sudden movements. Act like you don't even notice. Have a book or something with you to do and keep your attention on that.
Treats are also good for getting them to trust you.
If chasing or grabbing is necessary in order to keep them safe e.g. when you let them outside and have to catch them again if they won't come back willingly, then don't do that until they trust you and will come to you when called. Chasing and grabbing at them will just lead to set backs.
When I say spend as much time as possible with them, I mean literally. If you watch tv, have them on the couch with you. If you're on your phone/laptop, get the ducklings out and spread a blanket on the floor and lay down there with them. If you need to get a drink carry one with you to the kitchen with you. If you're preparing dinner then put some things on the floor that they can play with and let them free range the kitchen while you prepare your food (obviously be careful not to step on them and don't do anything dangerous like boil water etc). Talk to them a lot, ducks love to be talked to IME.
It's like if you had a puppy, you have to get the puppy accustomed to everything. The puppy will be afraid of all sorts of things, from different sounds to different textures to people and so on. It just takes time to introduce them to everything. In your case, you need to get them accustomed to YOU.
Biting isn't always out of fear or aggression - some ducklings just like to chew on your skin or clothes, like a kitten or puppy might. If you believe it's playful behavior just have a toy with you that you can give them to chew on when they start chewing on you. If it's out of fear just ignore it, don't give them any reaction at all.
You just have to be patient and understanding. I've raised tons of ducklings and admittedly my ducklings (and subsequently my adult ducks) are always very cuddly because I usually get them very young, spend almost 24/7 with them and because a lot of mine have been muscovies which are very calm in nature (I've raised dozens of pekins and also some mallards too though). However, I've tamed many other animals and it just takes patience, understanding and acceptance for who they are.
My adult ducks are extremely cuddly and friendly and love spending time with me, much more so than my dogs and cats. Not all ducks are like that but I personally feel that most are if raised with people who spend a lot of time with them. Especially breeds like pekins and swedish, who are generally calm and friendly.
