You need to dilute the puppy milk. I think double the water, but Chris will have to clarify.
Are you feeding them through a syringe? If you are, you can cause it to go down the wrong pipe into their lungs and suffocate them. You need to put about a spoonful in your hand (I used a baby food jar lid) and dipp their mouth into it, careful not to get it on their noise. They will start lapping it up.
Chris R is THE BEST THERE IS so she will have to clarify but she helped me until my joey passed away unsuspectingly.
Just remember, It doesn't work often and the chance you will loose these is high.
Also, neuter dad. They must have some medical issue that you don't know about but mom does.
After you loose a joey, you will never want to have joeys again but, look on the bright side, you have some wonderful pets and can get another pair to breed.