Yep, your blade mechanism is likely out of alignment....
Here is what you can do if you have a gray cassette tank with the wheels.
Flush your cassette multiple times with fresh water...
Get some rubber/latex free gloves and put them on.
You will see a blade cover that slides over the cassette blade and rubber seal...pull this cover about half way off and use it as a lever to unscrew the mechanism out of the cassette body,,,
You should see the circular design in the top of the cassette... This is designed to unscrew....
After you get it out the problem should look apparent to you...pop it back into place, test the blade operation by using the little green knob on the cassette... Clean up the blade seal and use dielectric grease to condition the seal.
Reinstall the mechanism into the top of the cassette.... It literally took me longer to type this than to fix the cassette.
Thanks!
Jeremiah