Osteospermums

Last year I planted Osteospermums in 3 different colours in the greenhouse border, they flowered really well and were 12-15ins high. I trimmed them back and left them to grow for this year.
They put on a lot of top growth and are flowering at 27ins tall  I tried the Chelsea chop on a few of them, when I could see how tall they were getting, but that only delayed flowering - they still grew as tall as the others.
Obviously I want them to stay smaller, any idea why they've grown so tall this year? The bed just got a sprinkle of BFB at the start of the season -- just as it did last year 

I don't think they're competing for light as they are at the front near the doors, so in full sun most of the day.
Exactly the same place as last year, perhaps it's just the weather we've had this year