Disappointed

I bought AGM battery which comes on 3years warranty in Old road Midrand branch. When I bought this battery, it was confirmed that it's AGM supporting the start & Go of my car but the battery stopped supporting that function just when it reaches 2yrs. At first I thought there was something wrong with my vehicle until I sent it to Mercedes for its service and it picked the main battery to be low. I went to battery center where I bought it, they checked the battery and said it shows that it's still healthy. They decided to lend me their old battery so to keep my battery over a week for charging. The vehicle start & go was functioning very well with the borrowed battery. I then returned their battery so to get mine and it's still not supporting this function. Honesty I felt so disappointed for the fact that the battery comes on 3yrs warranty but the way I'm getting treated seems someone is taking a ride on me. I turn to ask myself if it's because I'm a woman or what... I regret why haven't I just waited for Mercedes battery order instead of this struggle.. I've already been to the store 3 times because of this matter still with no luck. And first day when I got there I found another client with the very same vehicle made as mine with the very same complain that the start & go has no longer in functional. It seems this batteries are not having enough power for this function, that's why it doesn't sustain it long. I mean this start & go stopped exactly when the battery reaches 2yrs. I've even told them I never experienced such with the previous Merc battery. One guy wanted to shift the blame to Auxi****y and it's unfortunate that's still in good condition as I bought it the very same week when I buy this main battery. He even went extra miles on testing my Auxi****y and it was confirmed to be very healthy as it was already been diagnosed at the dealer but it seems they didn't trust me. What I need to understand is that How does warranty work if here I'm encounting a fault with your battery 🤔 Can someone advise on this matter as a matter of urgency please. I really can't be expected to gets stuck on this as if I am driving a normal car, no more comfortability in it. I really don't feel it if you understand what I mean... Juliet