Did you fully split the tubers or store and re-plant a large clump? Clumpy dahlia that aren't fully split can produce weaker, less supportive stems. It's the one disadvantage to my dahlia as I never fully split the things.

I'm not directly familiar with Oo La La, it may be that this cultivar requires staking or a tomato fence most of the time. I'm sure somebody who is familiar will be along.

It might have gotten less fertilizer, sun, space or water this year. I'd grow it again if you like the bloom, being sure to provide as much of those four as possible. If it was great last year and not so good this year, it can still come around to be great next year, since it doesn't sound virused.

Incidentally, many growers that are serious about showing dahlias put absurd amounts of fertilizer on their plants by using low levels every day. The leaves and blooms show the difference, and it is amazing how fertilizer can transform a variety you've grown for years into a work of art you don't recognize! Many have complicated recipes that they only grudgingly share, which have been tweaked over many years in their show gardens.

Watering daily during bud formation can increase size of the blooms, and I bet it would help stem strength a bit, too.