It's tricky to give a diagnosis but i suppose there are some basic things to check. Have a good look over it first. Any sign of broken mainspring, bent levers etc. don't force anything in trying to cock it.
Does the gun have any signs of missing or damaged parts, does it seem to move the piston back when you tried cocking it, did it move smoothly?
With their age & varying quality of manufacture wear or breakages are a distinct possibility but they are pretty simple mechanisms so taking it appart might be the best option to see what the internals are like. You can see the condition of the spring, try operating it without the spring to see if cocking it moves the piston back enough to latch or if not maybe the cocking arms bent, wear to the cocking slots a possibility.....just have a look in to see if there's anything you can see that dosent look right. A few pictures might help highlight something that's not right. If your not comfortable taking it apart yourself then maybe someone could suggest a local gunsmith/repairer local to you.