Lazy Autoflower

Hi guys.

This is my 3rd grow. This time went for Sohum living soil watered with ph’d butt water with Blumat drippers. Fool proof I thought but nearly 6 weeks in and it just didn’t have the veg growth spurt my other plants have had. It started flowering in week three and hasn’t bushed out hardly at all. Temp,lights humidity just right always. Could it be a bad seed, genetics or does this just happen sometimes. I’m a bit pissed because the soil just isn’t cheap and thought it would be the answer to all my nute problems.