How do I go about leveling out streak marks in my wood stain?
Have you ever used Rubio Monocoat?  It is an interesting an unique one application product - oil.  Solves issues like the one you are experiencing.  At this point, assuming your wood is properly dried and doesn't have some strange grain pattern causing inconsistent absorption, you can try another coat of evenly applied stain.  If it still is darker on part of the plank. either the grain is different, the stain is unevenly applied, or there may be something that seeped deeply into the wood causing parts of the wood to adhere differently.  If the latter is the case, ponly sanding beyond the absorption point would likely work.