Are they crowded? Do they have sufficient light? That could etiolate them and weaken the stems.

Not crowded, more light than lots of others that are stem firm, leaves a clear green.

I frequently have the same problem. The only one that do not flop are the small flowered ones like Bridal Crown or Cheerfulness. The big doubles are just too heavy for their stems. I wish someone would hybridize doubles with shorter stems.

The big doubles also have bud blast (where the buds dry out without ever opening) at least half the time. I have a big stand of one (I think Mary Copeland) I have never yet seen bloom. Bud blast every year.