Polaris 65 only goes backwards

[LindaY]

I have a polaris 65 that only wants to go backwards. I took the top off and watched while it was running and it looks like the randomizer tube starts to spin and go back into the connector chamber but then it stops and doesn't come back out. Do I need to replace the piston assembly or are there any other suggestions as to why this is happening. Thanks for your help.