1. If you put the belted cartridge into the middle of the feed tray with the bolt forward, close the feed tray, and then pull the charging handle back, of course it doesn't feed. That's not the way to load the gun. The bolt should be rearward when the belt is laid in and the top cover gets closed. Then you let the bolt go forward.
As a semi-auto, the gun now fires from a closed bolt, but it wasn't designed to do that. It was designed to fire from an open bolt, and to be loaded from an open (rearward) bolt, so the belt feed is made that way. If the bolt is forward when you put the belt in and close the cover, it's trying to advance the belt when it can't because the cartridge is against the stop in the feed tray. It wedges everything, and the bolt can't strip the cartridge. I'm guessing you didn't get or read the directions for using this semi auto gun? (Not trying to be a smart a$$, but this isn't an AK or AR or like anything else you probably have experience with.)
2. The gun shouldn't fire when a cartridge is dropped into the chamber and the bolt is allowed to "slam home", unless you then have to pull the trigger to make it fire, which is not what you said.
Reply in great detail, where the bolt is positioned when you put in the belt, and each little step in the process.