Oh......I thought you were supposed to pick them up and cuddle them.
I use the air flow teats which have a soft red teat and a yellow screw on bit. I slice the tiniest fraction off the end so the milk flows easily when sucked but dosent drown them.
I think she is a bit poor for some reason. Either she swallowed gunk as you suspect or was just born that way. Feeding her with the tube will fill her up so she may need to get extra hungry in order to take to the teat.
Sounds like you are doing all you can for her. If she dosent thrive it wont be for want of trying.
Must dash,
Pixie is three weeks old today and has moved up to a new cardigan size!