I'd recommend putting the hub together without the spacer. fixed-gear hubs usually only give you about three or four threads for the cog to bite onto, and the same for the lockring. You want to use as many of the threads as possible, and the spacer is just offsetting your cog away from a solid purchase on the threading.

If your lockring isn't at least flush with the face of the hub, you're making a big compromise. Take the spacer out and I think you'll find that the lockring threads on all the way to flush or even slightly further. This will probably solve your problem; I bet you only have one turn of thread holding your lockring on. That's a recipe for tearing that thread off and having all kinds of bad things happen.