NPT not threading/screwing in...

I am putting some all grain equipment together and recently bought a slew of fittings from bargainfittings.com . I had the bulkheads welded on to my kegs and was ridiculously excited to put it all together. But when screwing in the nipples and etc into the bulkheads, I get only a thread or two in before they lock. Is this normal??? I noticed a little slag on one of them so I took it back to the welder and he "threaded" them back using some tool. Still same problem remains! Thoughts, solutions?

Stainless threads can sometimes be a PITA because they tend to seal poorly.
But to answer your question, stopping after 2 turns is not normal. (Hopefully your wielder did not thread the nipple with a NPS tool)

No, he didn't touch the nipple, just the kegs/bulkhead. Apparently it's called a thread tap?

The fitting could have warped from the heat of welding. But if he can run a tap through the fitting it should be ok. Maybe it's your nipple that's bad. Stainless has a tendency to gall so don't force it. A properly cut thread should be able to hand tighten 3.5 full turns. Now that doesnt mean it won't seal of it only turns twice, I've even had threads over cut that I burried the fitting and it still sealed. I don't like screwing any stainless together with at least some Teflon tape on it.

One of two things happened (more than likely)...

1) You said there is 'slag' on the inside of coupler? That means he used a MIG machine and not a TIG machine to weld. Have him run a tap through the coupler and it should clear it out.

2) He welded the coupler too close to the threads and it distorted the whole coupler. He can try running a tap through but the coupler may be shot.