Some guesses, no guaranteed solutions. Check for worn sear notches, worn sear noses, weak sear springs(although if the sear point and hammer notch are both good it won't take much sear spring to hold it in place). Check for any of the above being gummy and for interference from the stock, holding up a sear from engaging fully. If someone has tried to "improve" the trigger pulls, they may have messes up the angles. These are not high quality guns, so honest wear is probably the most likely culprit. Let us know what you find.
