There's a variety of reasons that those plants could be stunted; lighting, humidity, grow medium, water, temp, etc, etc. The answer is to get it all right (or close) and keep it there. FB Blackberry is going to grow into a short, broad plant so there may not be anything wrong at all.
I think the key to growing autos is to take full advantage of the first 4-5 weeks and max out the veg growth; then there will be more 'plant' to sustain more flowers. I've been defoliating my autos, but only a little at a time, so as to not induce too much stress at once.