Well ive just had the same problem with mine tonight, I could only select either 1 & 2, 3 & 4 or 5 & 6, only two at a time. It was not good and I have had to put my standard linkage back on for now. Looks like ive had the same problem in that the bush is getting caught and eventually making it impossible to get a gear.
Im not best pleased but im hoping that VWR have a resolution but from what I can see where the bush slides the lip just isnt long enough as per the original one.
I will be in contact with them tomorrow to see what they can do, ideally I would prefer my money back as ive lost faith in it!