at 02:54 PM
We can rule out corrosion as the problem because 1928 isn't old enough for that problem and all zippers are nickel coated for resistance to elements. We need a little more information to really understand the problem but will take an educated guess.
Assuming it is the front zipper, there is a chance the head of the zipper has been bent and no longer engages the zipper teeth. When looking at the zipper head, there is an opening for the teeth to slide through. Check to make sure this opening is symmetrical, that all metal spacing is parallel. If something looks bent, take a flat head plier and squeeze the spacing tighter. Open the zipper completely, you don't want to fight with the teeth or get them caught in anyway. Forcing a bent zipper pull to work will only damage the zipper teeth. If the teeth become damaged, then you would need to replace the front zipper. Zipper pulls can be repaired, zipper teeth can not.
If the zipper head is working properly, you may want to rub soap on the teeth. Soap will help to lubricate the teeth to minimize friction and also will not harm the jacket. We prefer soap over oils because the oils can get messy and can get all over the jacket.
If you want a professional opinion, you can take your jacket to a professional leather cleaner; there is a list of specialists on the Leather Apparel Association's website at www.leatherassociation.com