Be sure to observe the marking on the pedals ("R" and "L" - left pedals have opposite threads). Reattach and tighten the pedals securely. Pedals are threaded in such a way the riding will tighten them, if this does not work it maybe due to wear in the pedal or crank arm threads. Pedals or crank arm threading may be stripped, if the threading is smooth then a replacement may be needed.