Janeee,
The thrust is always calculated and set so that you will be airborne at the end of the runway by a safe margin.
Something here has gone wrong. Either the aircraft was heavier than the paperwork said, or something else has happened to the same effect.
It is sometimes only apparent to the pilots that something's wrong when it's too late. They may have applied full thrust as a last minute bid to save the day, but it didn't work.
Hope this helps,
Stu