The scanner’s slide tray may be bent due to overuse or misuse or there may be something obstructing the tray slot.
If the tray is bent, take out the slide tray and check to see if the tray is bent. If it is, you will have to purchase a new slide tray for the scanner.
If there is something obstructing the tray slot; take out the slide tray and check to see if the tray is not bent or broken. If it is not, you will have to look through the tray slot to see if you can see any objects that may be preventing you from putting the tray through. If there is and you are unable to get it out through the slot, you will have to open up the scanner..