Boeing Starliner officials may know ‘root cause’ of issues with astronaut flight | CNN
www.cnn.com
After weeks of troubleshooting, NASA and Boeing officials say they better understand the issues plaguing the Starliner spacecraft, but still aren’t ready to name a return date.

tl;dr

Heat building up inside the thrusters may be causing Teflon seals to bulge, restricting the flow of propellant.