Had the same problem with mine mate
As Naz said just knock the edges of the upright bit in so they grip the sides of your shoes
Solves the problem completely
Jason

Thank you Jason, thank you Naz, I'll try to knock the edges of the upright, but now I have a another serious problem!
My left spring has been broken!
Do you know where could I buy a pair of spings for Powerstrider Olympic (I from Italy!)?
Naz do you have available?
thank you