727 No 3rd Gear

Well Saturday a friend of mine developed an issue that cost him the win at our local drag strip. Rebuilt 727 with many upgrades was purchased from a local and very reputable builder and installed in April. He has put maybe 30 passes on it without issue. Well Saturday, about the 5th round he said it wouldn't go into high gear. He said it when he tries to go into third it's like it's trying to go between two gears and feels like it's going to lock up.

What can cause this? Thanks for any suggestions.