I'm sure there is post about that elsewhere, I probably done one myself before. Something must be done for the level restrictions for the episode mission. I can understand the at the beginning there was maybe not enough. But now, there is. You should lower the level restriction. Put at least 2 at the same level, so they be enough to level up. I try to level up my 3th Starfleet character. But I do one mission, must do something else, Starbase 24, fleet mark missions or anything else, a few of them to get to the next level and do another mission. With at least 2 per level, I would probably be OK. Anyway, right now, many are forty something an more and the thirties are a pain. Make some start at lower level and group more than one per level, please!
I had this same problem (I skipped the tutorial, and ended way behind the level curve). I discovered that picking up a random system patrol would usually put me at or over the level - I'd often get 2k. Especially with the dribs of exp that doffing provides, once I discovered those system missions, leveling got a whole bunch easier.
People already level too fast in this game. It's good that they have to take some time to do things that might actually teach them something about the game before they level up. And tbh, story missions are pathetically easy. So doing things outside of story missions are a good way to learn.