In a semi-auto AK there's really only three parts that can cause this...the hammer, the sear, the disconnecter.
First step is to remove the receiver cover, pull the trigger, pull the bolt back, and ease it forward while you watch what's going on.
With the trigger pulled, when the bolt moves back, the hammer should be caught by the disconnecter.
When the bolt moves forward the hammer should be held by the disconnecter until the trigger is released, at which point the hammer should be caught and held by the trigger hook.
Look for.....
Broken, worn, or damaged disconnecter in the area that engaged the hammer.
Broken, worn, or damaged hammer shelf where the trigger hook engages.
Broken, worn, or damaged trigger hook.
Fouling, bits of steel cases or other debris in the trigger assembly.
Rusted, broke or other wise damaged sear spring.
Free movement of the trigger and sear and good spring tension.
Broken, rusted, hammer spring, or hammer spring legs off the trigger.