after i left the hoover damn because i didnt want to beat the game because it wouldnt let me keep playing after i just left and ever since it wont let me fast travel but every once in awile it lets me do it once its pissing me off because i love fallout so much but im tired of walking everywhere

Did you already start the last quest though?

Did you choose to continue when you were warned that there was no going back?

If so then that's why. Once you have started the last quest you are not supposed to go back. That's why you can't fast travel.

oh ok thanks