Endive lettuce

I planted a load of endive seedlings in early May.  They have realy come on over the past few weeks, but dont seem to be hearting up, and a few are bolting.  hard to describe, but they have really spread out leaves but no hearts, and I'm definetely seeing a few of the dreaded bolting spikes appearing up the middle.  Ive never grown endive before - am I doing something wrong?  My other lettuce varieties are doing great in identical conditions.

Hmm, figures.  Just went by the seed packet recommended sowing dates   At least I now have space for my kale and PSB!  Thx.