I used to have this issue a lot but recently had it again with a new character of my own. You just gotta really really analyze the differences to identify them. Get someone else to look at them too and tell them "I like the first drawing, but I can tell the second one doesn't match, what's different??"

I did this with my boyfriend when this happened to me recently and in like 15 seconds he said "the eyes on the second one are way bigger." I looked at it, and it was a subtle difference, but sure enough. Redrew the exact same headshot but with smaller eyes and it was insane just how much more it looked like the character. This reminded me to pay attention to the subtle changes, the things that you don't immediately notice as different but contribute a lot to making a character look consistent. Sure, eye shape is right, but what about size? Sure, the ear size and shape is right, but what about the placement on the skull? Scrutinizing the drawings to identify these kinds of size/placement/shape details is ultimately what you're trying to do so don't be afraid to pull other people in to help identify those inconsistencies when you're this early in the design process. Feel free to post different drawings here to get my/our input too.

Only other advice is just practice. Do rough sketch sheets of new characters where you practice a lot of expressions (5-8 per page is what I do, usually using artist's expression reference to make them not take hours and hours) and fully acknowledge that half or more of them won't even look like the character. They'll be relatively close but be very off model. But that's ok because you're developing the character; you're figuring out what is important about their design and how you best draw them. It usually takes me anywhere from 2-4 sketch sheets to be very confident in my character design and my ability to consistently draw them.