Does the projector run in reverse?? If so, then what you are facing is a timing adjustment.
If it does not cycle in either direction, then there is a high probability that the solenoid "link" is broken. The "link" connects the cycle solenoid plunger to the slide change mechanism.
The good news: There are still parts available (www.micro-tools.com). The less-than-good-news: both fixes are difficult and best handled by a factory-trained repair dealer.