I think you will find all own brand composts are just soil improver from the council garden recycling plants. It is hit and miss if you get anything decent. I had two bags of the Verve stuff both totally different. The beans I put in one tray rotted, but as I started a new bag the other tray grew properly. Both in the same greenhouse with the same treatment.
They cannot be leaving it long enough to mature and rot down properly. Just brown stuff, be it wood pulp, xmas trees, or whatever, then a handful of chemicals chucked in. I go to the tip sometimes and get a trailer full, I know the bloke and he points me to the oldest heap and although I usually spread it on the plot some of it is far better than the bagged up 'compost' that I buy. The big names have got into the act and have raised the price of their products as they bung a shovelful of peat in each bag which makes it look nice and crumbly.
I tipped another bag out on a small raised bed I was using for seedings. All it did was mulch the area and stopped things growing.
