MX8 POOL CLEANER NOT PERFORMING

We bough a Zodiac MX8 for our pool last December. Before using it we fibreglass the swimming pool and got professional Pool Technician's to install a brand new pump system, including replaced all pipes ! Now, we bought this cleaner on good merits, but ever since it got into the new pool it only works for a while and then I have to take it out and turn the wheel to that it could start. I spoke to a technician after it came back from the workshop, he told me that a Pool cleaner is only there to keep a pool clean. I understand that but come on, there is not much dirt in the pool, but the sucktion is there but the MX8 struggles to move so that the dirt can be picked up. He told me that when they test my MX8 it worked, but it was in their clean swimming pool !!! Why would anyone buy a cleaner if there is nothing to clean? I want a replacement please, I am not sending it to the workshop again, because I had to pay twice for the courier. My MX8 is not working, please replace it with a new one, you guys told us at Somerset-West Builders Warehouse that you would replace it if we have any trouble, so we still have a big problem, IT IS NOT WORKING !!!