Is the bolt locked up into the barrel extension with no cartridge chambered, or is it locking up with a cartridge in the chamber?
If you get it apart, clean the chamber/locking lug area (back side of the lugs in the extension, and also the locking lugs on the bolt.
If you are familiar with lubing the lugs on a bolt action, would suggest lightly lubricating the back of the lugs on the bolt and seeing if this get better after you clean and lube the lug area. There could be burs or debris in those areas that are effecting lock-up.
If the problem persists it could be a tolerance problem on the machining of the barrel extension, the bolt or a combination of the carrier and bolt causing things to lock up. Possibly why the asked you to try a different bolt or bolt and carrier.
I'd try the different bolt in the current carrier, followed by a different bolt carrier and different bolt combination, as that could isolate the issue. If it all still is way to tight, there could be a problem with how the extension was machined or installed on the barrel.
If you have a set of go/no-go gauges you could also confirm they headspaced the extension to the barrel correctly.