my rubberized ball cover on the right side keeps coming off....is it defective? I take the back...

end off and think it's fixed and then I put the machine back together and it keeps slipping off?? Do I need a new rubberized ball cover? I have a Pro Form XP Stride Climber 600. Please advise and if it is the ball cover who or how could I order a new one? Thanks for your immediate attention!

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