Needs adjusting, you need to have play in the top of the pedal.
You can adjust the biting point down by the rod on the back of the pedal. This will stop it slipping.
You need to loosen the retaining nut then turn the rod anticlockwise. Keep checking it as your winding it, you will feel there is less tension at the top of the pedal. Once you have done that give it a try, you want the biting point in the middle some where if possible.
Keep adjusting till its right, if you go to far the bite point will be too low and you won't be able to get it into gear.
Once you have finished tighten the retaining but back up, job done.