I've most probably had the same problem and all of the others that can be had with Dahons but we won't get into that now or I'll get angry.
Assuming you were a fool like me and bought it from a source other than a proper Dahon dealer you can't take advantage of their 5 year warranty.
If that is not the case and you are still inside the 5 years, make haste to said dealer and don't take no for an answer.
If not you have some options;
Firstly, adjustment which has already been mentioned {tighten/loosen exposed bolt head in latch INCREMENTALLY until it is right}.
Secondly, replacement of all or part of frame latch via a Dahon dealer {who could theoretically make a fraudulent warranty claim to obtain the part and charge you}.
I have done this many years ago and was charged $800 for an entire latch the bolt of which broke within a week.
If I remember correctly I replaced the internal screw of the bolt the bolt myself the 2nd time.
Finally, check that the mating surfaces of the frame do in fact mate correctly. Mine no longer do as a result of constant problems with the hinge caused by actual RIDING of the bike.
If not, think about how willing you would be to be in front of that bus again and you'll probably arrive at the same conclusion I did and scrap it.
For those interested my bike was a grey market import from China bought via the internet which I got about 5000km out of and anything else I could say about Dahon would probably get me sued in Taiwan so I won't say anymore.
O.P. P.M. me if you want a lead on a shop in Taipei and send me a pic of the part {which is how Dahon do it 'cos they don't have part numbers!]