In that case, field strip and remove the grip panels. Remember, take out the mag first. This will expose the trigger guard pivot pin. You can remove this pin with a punch or a nail with merely the pressure from your finger. Once removed, the trigger guard falls free of the frame. Then, use a flashlight to inspect everything.
There are a few things that could be going on. The parked finish may have built up to heavily in several places. First look the removed trigger guard over well. Pay attention to the area of the trigger guard that fits up into the frame under the barrel. If there is excess build up in that area of the trigger guard it will not lock up. Second, look at the area of the trigger guard that fits into the frame in front of the magazine well for too much material. Third, check the area of the frame under the barrel as well as the trigger guard pin mounted through the frame under the barrel. Too much parked material in this area with prevent lock up. You are looking for any excess build up from the parkerizing process.
Fourth, the trigger guard pin can be either part #16 which is a solid steel pin or part #40 which is a roll pin. If the slide stop lever spring is bent and in the wrong place or it's the wrong spring for the wrong pin, that can prevent lock up. Take a look here at this diagram which has both pins pictured to see which one you have
http://www.czub.cz/zbrojovka/cz-spare-parts/cz83_r_en.pdf. From what I understand, the roll pin is the newer modification which requires a different slide stop spring. If the pin was swapped out when the gun was parked but the spring wasn't replaced that too can prevent lock up. At least that's what I understand from other discussion threads.
Fifth, the trigger guard may be bent or warped. Lie it on a flat surface; there should be no wobble. If it wobbles, it's warped a little causing misalignment. Next with the slide removed so you can see, pit the trigger guard back in the grip frame and reinsert the pivot pin. Try to lock the trigger guard in place. If it too long to move by the pin and lock up, the trigger guard has been bent in the middle enough to cause it to be elongated and out of specifications. With a vise with padding and some care you can fix either by bending the trigger guard what ever way it needs to be bent in order to correct that problem.
Those are the only things I can think of that would cause the problem you're having. I hope one of these ideas helps.
