transmission problems please help

[4seasons69]

ok so I finally got done putting in a transmission that I was told was rebuilt about 21000 miles ago. it is a manual. so we go to take it for a test drive I put it in 1st and give it a little gas and start letting out on the clutch and it goes no where. it kinda sounds like the gears are touching and spining but not fully engaging at least that is the best way I can describe it. then I put it in second and it goes. every other gear works great and it feels like it has been rebuilt. could it be something with the shifter cables or is 1st gear shot? anyone have any Idea what this could be?