There are several possible causes for this issue.
A. Cartridge Position. The slide cartridge may not be fully inserted into the printer. Push firmly at the bottom of the slide cartridge to make sure it is completely inserted into the printer. You should hear a definite click. Some slide printers will click louder than others. If you can pull the cartridge out without lifting up slightly first, it was not fully inserted. Note: It is possible for the printer to register the presence of the slide cartridge without the cartridge being in the correct position for feeding.B. Sticky Slides. If the slides are sticking together in the cartridge this can prevent the slide feed roller from grabbing the slides and feeding them into the printer. Put one slide in the holder, if it feeds properly, you know the problem is sticky slides. The easiest way to resolve this issue is to try different slides. Humidity is the main cause of slides sticking together. The following factors can affect humidity:
Note: To determine if your slides are sticking due to oil, dirt or water between slides, press two slides together. If the slides are clean and dry the slides should appear completely clear when you look through them. If oil, dirt or water is present you will notice refractive patterns in the glass. To simulate an extreme case, breathe on the slides like you would if you were cleaning a pair of glasses. Immediately stick the slides together. You will see refractive patterns due to the water vapor AND you will have a difficult time separating the slides.
C. Slide Thickness. The slide cartridge will not accommodate 100 slides of all types. Some slides are too thick. It is possible too many slides are inserted into the cartridge which is causing friction and preventing the slides from feeding. Try removing half the slides to see if it improves feeding performance.
D. Dirty Slide Feed Roller. If the slide feed roller gets dirty it can prevent proper feeding. Clean the slide feed roller with alcohol. Manually turn it as you clean to clean the entire circumference. The black slide feed roller is visible at the bottom of the slide holder cavity when you remove the slide holder.