Honey, I am writing you because I had a feeding tube. But mine was in my stomach. I have had a tube in my nose too. The main thing you need to try is giving small amounts and then waiting to see how she handles it. You are putting too much on her stomach at a time. Her stomach could have shrunk in size. I am a nurse so I do know some things about NG Tubes. Try giving her something for her nausea before you give her the Ensure.
Her Dr. should have prescribed something for the nausea by now. If she can't take it orally give her a rectal antiemetic. They work. Had my feeding tube out last month. Things right now are getting better for me but it took sometime.
When I first got the feeding tube I didn't think I would ever get any better. Talk with the nutrionist that should be on her case. They can give you suggestions too. Prayers are with you.