Garden tools not strong

I bought a Hoe 3 prong and hoe 4 prong for my garden on the 10 October 2017 and when I tried using both they just bend,you cant do a thing with them,I tried returning them at the shop to at least exchange for better ones and the manager told me there's nothing he can do and its none of his business that the prongs are weak.I did not expect to get such service from them as I am a regular customer there.