Probably got a bit too big for their own good whilst other chains improved. I've seen that they still have lots of completed stores they've acquired but haven't actually moved in to and their rapid Express expansions aren't always viable in some areas as they try to compete to heavily with others practically on the doorstep. In a place called Summertown in Oxford; we have a Tesco, Sainsburys, M&S and Co-Op all within a 200 yard space. It's just mental but if Tesco actually moved half a mile up the road near a prime housing area; they could have made a killing instead of trying to drive others into the ground including themselves. Also their product range is getting a bit too mainstream, predictable and lackluster whilst the likes of Aldi and Lidl seem to be coming in with more original goods more ideally priced.
Tesco need a range refresh and overhaul and need to get their prices competitive again. In the last couple of years; they've taken the market and people for granted and perhaps have been far too cocky and arrogant. Others like Morrisons are even catching up by driving prices down. Christmas is gonna be a crucial time and I think they'll struggle as it's not enough time to dramatically turn the tide.