You might want to join the wefixit group on Yahoo. They specialize in sewing machines:
https://groups.yahoo.com/neo/groups/wefixit/info
Most of those mechanical machines work on the same principle, so likely someone there can tell you what to look for to correct that problem. If I had one of my mechanical machines out of storage, I could probably figure it out for you. Were you able to take off the cover on the left side of the machine? That is probably where you need to be looking. Usually there is some kind of a lever mechanism that clicks into place when you raise the presser foot. It may be that a screw is loose, or it needs oiling in that area, or a piece broke off.