Firing pin problem?

[Rocky Raab]

I agree, Fabs.

You could have a speck of crud between the barrel and the receiver shoulder, or the barrel may not be exactly indexed. If it's rotated just a wee bit out of alignment, the extractor hook may bind in its barrel cut just enough to hold the bolt back a tiny fraction.

Third, its possible that the barrel is cut wrong.

Take ther barrel back out and clean it and the receiver (plus bolt) well. While it's out, measure the extension of the barrel, from the shoulder to the breech, and compare that to the factory barrel. When you re-assemble, make very sure the barrel doesn't get rotated and that the extractor enters its cut cleanly.