I just used super glue with the washer. Then the magnet just sticks to the washer without glue. I stack two magnets. Also, my fan is pretty weak too, but it's still enough to spin the stir bar in 1500 mL.
In Vino Veritas, In Cervesio Felicitas Anonymous
Hmmmm...I used hot glue on mine and it's held for almost three years now. I know there are different types of glue sticks (heavy duty versus general purpose) so maybe try a different one.
Regarding the fan power, how many volts is your power supply? Could be that your fan is worn out if it came from an old tower. I only run 6 volts through mine because 12 was way too much power. It has quite a bit of power though...I wouldn't voluntarily try stopping it with my finger.