Sounds like you got some crud in the threads in the cylinder. If you don't have an M6x0.75 tap, take a .22 cal bronze brush and carefully clean them from the back of the cylinder towards the front. A bit of bp solvent on the brush should help. Then use the brush to clean the nipple threads as well.
If that doesn't help, take one of the nipples that did screw in and try to start it in the hole.
And be sure to use some anti-sieze on the threads.