VW Golf VR6, '98 - 5th gear problem

I have another problem with my VR6. For some reason the 5th gear (manual transmission) does not want to go in. Sometimes it does go in with no problem, but recently, most of the time it just won't. This started happening about four months ago. All the other gears are fine. Any suggestions what this might be caused by? Is there any regulators on the linkeage? Any help is appreciated. Thanks.