Neil another trick can be not to be sitting up too straight as that can put pressure on your stomach.
Sometimes to get the flow started I would have to just kind of hold the syringe fairly high and get Chris to force his stomach up and down a bit and lean back a little.
It's so hard to describe with text, I can see the little tricks in my mind as I type but can't find the words lol
I never plunged the feed in, but make sure you also start just with water. Water is thinner so more likely to get the flowing started. As the water is halfway running out, add a bit of whatever you are using for the feed, ensure or jevity or whatever, so it is still a bit thinner and the flow is starting. Then you can lower the syringe a bit and move to the full thickness of the feed.
Hope that helps!