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.
Boeing:
https://sh.itjust.works/pictrs/image/969f5d56-4c7c-4e46-a462-23d67a5f2ef1.gif