Make sure it is not too hot either get a thermometer and in the mean time test the formula in the inside of your wrist to make sure it is warm, but not too hot.
Wrist testing is not advised. Too easy to make mistakes. You can buy thermometers in lots of places, supermarkets, chemists, drugstores. Can easily source one same day.
Make sure you trigger a feeding response before feeding them so the food does not go down the wrong pipe. When you trigger a feeding response, the bird holds its breath and bobs its head so the food does not go down the wrong way. If there is no feeding response, they may aspirate.
That´s right.
Forcing the beak open means they are not accepting the feed willingly.
Things to look at,
Correct formula temp - 105F / 40.5C
Use the same formula as the breeder did
Use a spoon
Mix the formula well, make sure no lumps or bumps
Keep the formula warm in a bain marie while you feed
Are the chicks housed some where warm? (edit; after looking at the pics, these birds should be inside a brooder when not out being fed)
A tip to try is dimming the light and feeding the chicks in the brooder.
What schedule do you have, and how much formula are you feeding per feed?
How much do these birds weigh? Are you weighing daily?
If these birds lose weight quickly, they will need to be gavage fed - you will need the vet to do that or teach you how to. Do not attempt to do it from watching videos alone.