Is it tight? If there is no movement when you wiggle the revolver, barrel in one hand and the frame in the other, it's probably OK. Make sure the hinge screw is tight, so it doesn't give you a false reading for tightness.
Check to see if there are any burrs, or built up crud in the latch where the tooth engages. Make sure there are no burrs, or debris in the frame cut in front of the latch, keeping it from going all the way forward.
Someone who may have taken the latch out may have dinged the frame cut or latch so it won't close all the way. Is the spring still pushing in that position? Some people remove the hammer block safety inside, and may have reassembled it with clumsily, and dinged something.
When the hammer is in the fired position, it prevents the latch from opening, make sure it still does.
If you're afraid to remove the sideplate and look inside, have a gunsmith do it and check the mating parts. The latch must be removed to allow the sideplate to be removed. Getting the latch back on is harder than getting it off. The latch spring must be engaged in its notch while moving the latch back into position, hold it in place and re-insert the screw.
There is also a trick to removing the sideplate. DO NOT PRY IT! With the screws removed, rap the frame on the sideplate side with a leather or wooden mallet, the sideplate willl pop up.
Replace it carefully, don't damage the SHARP edges.