Help Please.

I have a daihatsu Fourtrak which all of a sudden wont change down from 3rd to 2nd when I am rolling. If I stop at a junction put it in neutral then it will go into 2nd or 1st. But not when I approach a Junction and try to change down it just will not go into 2nd or 1st for that matter. Has anyone else had this issue and I dred to think what the answer might be.

Thanks John