LT77 gear selection issies

[MGBracer]

Hi All.
LT77 Issues. Does anyone have any idea why the LT77 box will not engage 1st 3rd and 5th, but will engage 2nd 4th and reverse?? There is no fouling of the shifter and when the stick is pushed all the way forward there is a just a horrible noise of the cogs meeting but not engaging. I've checked the bushes and the same issue happens whether the bushes are in properly (new bushes) lossened off or removed and nipped up to the box. I'm thinking it could be a selector shaft issue in the box?? I hope someone can help. Cheers.