Salad leaves running to seed

I am growing several containers of salad leaves, each sown at a fortnight interval.
However, the latter sown seedsseem to have caught up with the earlier sown seeds resulting in some of the salad leaves running to seed before I have had a chance to pick them.

Does this mean that when I cut them they will not re-grow?
What can I do to ensure this doesn't happen?

Any adive to this newbie would be gratefully accepted.

Thanks

 

I'm guessing that it's the weather.
Most types don't like heat and really don't like to dry out at all.

I believe that if they have run to seed (bolted),they are unlikely to be much use.

 

I believe it depends on the lettuce variety either. Some packages have the data if the variety is resistant to bolting, or if it can be successfully grown in summer.