Don't think my twins like the taste of Enfamil Nutramigen?

My twin girls are just over 11 weeks old. We are now on our third formula. first enfamil lipil with iron they were throwing up a lot on that. im talkin projectile. then the dr recommended enfamil ar lipil. they did a bit better on it but became constipated and would cry during feedings. now we went to Enfamil Nutramigen. We started this on friday evening and since then the crying while eating has stopped and they are having great bowl movements. But now I'm having a hard time getting them to finish their bottle. we were up to 8 oz of AR and now with the Nutramigen they will hardly eat 4oz. seems they do good at first but when it is time to burp i burp them then try to go back to feeding and they start to just squirm around a lot and keep pushing the nipple out with their tongue but they still act hungry when i take the nipple out of their mouth. so i put it back in. same thing. at that point i can maybe get another oz. down. but it ends in me being very frustrated and sometimes them too and then they aren't eating enough. Their entire routine is messed up. They went from 8 oz and sleeping about 6 hrs at a time. to 3 to 4 oz sleeping only about 2 hrs. What is going on? is it the taste??? because it smells horrible. i have read about other people mixing the previous formula with it like half and half and decreasing it gradually till its just Nutramigen so they get used to it. is this ok to do? or will it make them sick? Im very happy they don't seem to be in pain anymore when they eat. but now they don't seem to be eating enough. Any suggestions would be very helpful!! Thanks so much!!!