I had a really good appointment with my LC and hopefully maybe some of her tips will help you!
My DD has issues sucking because she just hasn't worked up that muscle tone in her cheeks yet, so while she's sucking she isn't doing to properly and not getting enough milk. So we're now re-teaching her how to suck properly. We're using a finger line and I just express and then dip the line in her bottle and put it in her mouth along with my finger. It's easy to tell when she isn't sucking properly, so I just have to "pinch" her cheeks in until I can feel she's really sucking hard. I'm sure there will be some information online somewhere!
We also need to do some gentle massage on her cheeks and that area under the ears/cheek that tenses up when stressed, just to try and relax her mouth to make sucking easier.
Do you have access to a LC? I was dealing with a private one but she was horrible, so my MCHN referred me onto the local council free service one and she is amazing. Very supportive, spent about 1.5hrs with me today checking everything and giving me tips.
It seems to be a vicious cycle... If they're not getting enough milk/calories they can't feed properly because they don't have enough energy... But they don't have enough energy because they're not feeding properly. It's so hard