Hello, welcome to JustAnswer.
Make sure you don't have a plugged tube. There are bugs that love to build homes in there. You may also have a little pebble blocking the tube.
Finally, it may be that the impeller is in need of replacing. They are an expendable item. Age alone is a requirement for replacement and when they are not used for extended periods of time the rubber blades can become permanently bent. This reduces the pumps ability to pump as much volume. The first thing to loose water is the Pee tube.
Try holding the flushing adapter tightly against the lower unit while the motor is running. If this causes the pee tube to start showing water, replace the impeller.
No water was coming out thru the propeller before? If not you need a new impeller.
How long did you hold it like that? You can let the motor run when holding the flushing adapter tight like that even if the impeller is bad as long as water is coming thru the propeller it is going thru the motor, so it wont over heat. It can take a minute or so for water to begin flowing out the site tube. If you held it longer than that, then you will need to find the blockage in the hose that feeds the pee hole. The blockage can be all the way back to the block fitting the hose is attached to.
Some times simply blowing compressed air back thru the hole will unclog it, If that wont work, you will need to disconnect the hose and fittings back until you find the blockage and clear it.