There are myriad of possibilities, but the majority of the time it has to do with an obstruction by some material of the engagement of the sear to the hammer.
The only proper way to assure what is the problem is to remove the stock and observe the internals. By reinstalling the trigger plate to the receiver in close approximation to how it sits with the stock installed and observing the engagement of the sear to hammer, etc. It's also possible that the sear spring has cracked or weakened and is only pushing the sear up high enough to just barely engage.
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