Bosal AK4 trailer coupling goes in harder and harder

Hello! I get my removable AHK harder and harder. Unfortunately I used it quite rarely (was most of the time in the trunk), so the warranty is already around.http://www.bosal.de/…/Default.aspx The last time I took half an hour until it worked out. resin/acid-free fine mechanics oil didn’t help. The silvery metal part, which should be shot back if you plug it in, is then flush with the actual black AHK instead of a black AHK. The status is of course not green, which is why I can’t get the key out. Did someone already have the problem and can tell me what to do? Thanks! notting