Figure out a way. Like when you write your hero into a corner then have to get him/her out. Do something no one else has thought of. In the TV show “Kung Fu” there are fadeaways, not really flashbacks, to show Caine’s past experiences (sub-plot) with the Shao Lin and how those experiences influence how he proceeds in the (present) plot. Write the flashbacks like a intro to each new chapter, like the intro to an entire book.