If I'm right, you're talking about the ribbon reverse?  First, does your ribbons have brads on the ends about 4" from where they attach to the spools?  If not, you'll need to tie knots in the ribbon about that same location on each ribbon about 4" from the spool so they will trip the arms necessary for the ribbon to change the takeup side and the supply side.  Be sure you thread the ribbon in through both trip arms so it will work effectively.  If what you have is a feeding problem, you'll have to see how the ribbon is advancing, even if the takeup side and supply side spools are about halfway.  You may need to do some cleaning around under there so the mechanism is free to move from one side to the other.  Most of those portables, and some uprights, use a sort of ratchet-pawl mechanism to advance the ribbon.  Without knowing exactly what's down there, you'll have to see exactly what both sides are doing in regard to advancing the ribbon.  To switch from side to side, you can either trip one of the arms, one side or the other to make it change direction, testing that feature out as well.  Or you can find the long rod that connects both sides of the ribbon mechanism that switches from right to left, and vice versa.  I hope this helps.

Royalist wrote:

What are "brads"? I don't know a whole low about typewriters.

Brads are little bitty brass button-looking things ribbon makers punch into the ribbon about 4" from where it attaches to the spool.  This is done on both sides.  If your ribbon doesn't have these, and your typewriter has trip arms (you'll see them--they're metal tabs just at where the spools sit on either side.  They move backward and forward, you can't miss 'em).  Again, if those brads are missing, tie you some knots about 4" from where the ribbon attaches to either spool, and make sure that you roll the spool inward somewhat so you don't get the knots caught on the wrong side of the trip arms.
 

Now what you'll need to look at is what each side of the ribbon advance does and how it does it compared to the other side.  Sounds to me like you might have some gunk down there not letting the ratchet mechanisms fully engage.  Or you might have something bent out of place down there for whatever reason.  Anyway, look at the way both sides function in the takeup mode--both right and left sides.  It could be possible that a tiny spring popped off, not letting the pawl or pawls engage the drive gears below the ribbons.  Let me know what you find out.