Sounds like the "Drum Fixing Pin" - Item 20, in the end of the selector drum, has come loose again. Your symptoms are the same as mine started when that came loose. Perhaps the bloke who installed it neglected to use Loctite. You can get access by removing the little cover with three bolts under the front sprocket. From memory the water pump and front sprocket are in the way, necessitating removal. If it was me, second time around, risking unnecessary complication to the job, I might remove the detent disk Item 19 to consider degreasing and applying Loctite 620 to the dowel pin Item 9, where it fits in the selector drum, to eliminate any play. Then apply Loctite 242 to the threads of the "Drum Fixing Pin" after degreasing with brake cleaner. Mine has not given trouble again since I reinstalled it in 2017. It has gone about 30,000 km since then.