If you got the couplers from MakerFarm, you have to really really tighten the end holding the threaded rod. When it feels it's bottomed out, keep turning it. I had the same issue until I kept tightening it beyond what felt like the bottom of the bolt hole.
MakerFarm 8" Prusa i3v
RAMPS 1.4
0.4mm E3D v6 for 1.75mm
