Both my kids are like that. Took weeks for my first kid to finally drink properly, even with the appropriate sized teat. So she, in a sense, starved quite a bit during that time (no choice, since I was at work). But later on she put back on all the weight. Second kid is going through the same thing now. Still struggling. Some lactation consultants offer bottle training. You can consider that. On my end, we’ve upsized the teat, and my husband is just trying his best to feed baby whatever she will drink.
Maybe the holes in the nipple are small?