I haven't had that problem, and I don't know of any fix for it, other than to reinstall the 757.

Try reinstalling the 757-200 Base Pack following these instructions so you preserve your mods:

1) Rename the FSX\SimObjects\Airplanes\CS_B757-200 folder to CS_B757-200bak
2) Reinstall (repair) the 757-200 Base Pack
3) Rename the FSX\SimObjects\Airplanes\CS_B757-200 folder to CS_B757-200bak2
4) Rename the FSX\SimObjects\Airplanes\CS_B757-200bak folder back to CS_B757-200
5) Start FSX and see if the problem is solved
6) If it is solved, delete the CS_B757-200bak2 folder