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:

1. Desiccant. Ideally, slides should be packed and sealed with a desiccant. The desiccant will absorb any humidity that was present in the air when the slides were packed. If the slides are packed by the manufacturer without the use of a desiccant but are sealed they can work just fine. However, there could be variations in the feeding performance of slides purchased from the same manufacturer depending upon the humidity levels of the environment in which the slides were packed.
2. Storage and Use Conditions. If the slides are stored unprotected in a humid environment they can absorb water which will cause them to stick together. If slides are removed from sealed packaging and stored in the slide cartridge unused for several days they can still absorb water. Therefore, it is also important to use the slide cartridge in a humidity controlled environment. Optimal conditions are 72 degrees Fahrenheit (22 C) and less than 50% humidity.
3. Charge Coating. If there is a charged coating on the slide this can increase the hydrophilic nature of the slide so that it is more likely to absorb moisture. Charged slides have the potential to stick together more easily.
4. Handling. It is best to handle the slides with a gloved hand. This prevents the transfer of natural oils from your skin onto the slides.  These oils can cause slides to stick together.
5. Loading. To prevent slides from sticking together during printing, before closing the lid to the cartridge, separate the slides by running your fingers over the edges of the slides. If you encounter any slides that cannot be separated, remove them from the cartridge and discard them. See this slide loading video for instructions.
6. Compressed Air. To prevent slides from sticking together during printing you may also blow compressed air (canned air) along the edges of all the slides after you load them in the cartridge but before you close the lid and engage the cartridge. Press the nozzle against the edge of first slide, engage the trigger and move slowly across the edges of all 100 slides. This 10 second process will force the slides apart and prevent them from sticking.

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.