Questions relating to refunds should be directed to customer services.
Without details around what part of the bench will not lock into place, or photos of the issue its hard to provide a detailed answer, however in this instance I can provide some generic advice. Should you be able to send me a picture of the bench I can offer further assistance, however for now would provide the following advice.
1. Start by checking the struts going between the legs are not bent or out of shape
2. With the legs deployed, ensure they are level and square
3. Check the pivot points and mountings holding the struts and legs in place are sound and not come adrift.
4. Where the struts, legs or pivots will not move apply WD40 or other lubricant to assist with easing the movement.
5. Ensure the legs are fully extended and struts level, to ensure the bench is 'locked out'
Do come back to me should you need anything clarified.