T140 Gearbox Outer Cover Problems

Hi
I know this is a question that has been asked a thousand times, but I have a '76 T140 that I can't seem to get the gears to select properly after removing the gearbox outer cover. I've read all the other questions submitted but don't see one for the issue I've got. Took the cover off to replace leaky gasket and now it won't downshift from 2nd to 1st. I've done this a few times before with no issues , so don't know what's wrong this time. I've lined the quadrant up in 1st gear , tried neutral and even top gear, but same problem. The gears go up and down with bike on stand but not out on the road, it select all gears smoothly on upward shifts and down to second, but not to first although starting in neutral it goes into 1st but after changing up won't go down again. I've even replaced the crossover shaft for one I had laying around, checked the quadrant plungers, springs , loosened the camplate plunger etc. Hope all that makes sense and someone can help. Thanks in advance
Paul