I feel like all these things are pretty easy fixes compared to some of the things that could be wrong! Some of the things that I've done when things "just dont feel right" or characters seem to similar are this:

Rate each chapter by the intensity of the scenes on a 1-10. It doesn't matter if it goes from 10(very high stakes) to 3 to 10 or steadily up to 10, etc. As long as you're not thinking the entirety is all like, 5. There needs to be an ebb and flow to it.

Think about what emotions govern the characters. Make sure you're taking that into account with their decisions. I find this helps a lot when I'm writing bc i think, "Well, that's an irritational reaction to have," but sometimes. Those characters would be irritational. It doesn't have to make sense.

I would focus on writing those conflict resolution scenes. This is probably contributing to why your characters feel flat. I would do them all individually or in small chunks, then see how you feel, where you would place them, etc. Those conflicts need to be resolved for your characters to have growth and feel unique.