Acoustic guitar, 5th string 9th fret has no sustain?
*Check the string. The fret(s) may have worn a flat spot on the underside. *Check the neck relief. Put a capo on the 1st fret and press any string on one of the highest frets. There should be a whisker of a gap between the string(s) and the top of the frets at the midpoint (7th fret?) If there isn't, you don't have enough neck relief. This is a truss rod adjustment that's best made at a shop. *Check the frets: Put a capo on the 9th fret and check the clearance of all the strings over the 10th and 11th frets. Is the A string lower than the others? If so, why? Examine the 9th fret for wear, and the frets above it for lifting or high spots. You may need to bring the guitar to a shop to get the neck relief adjusted, frets leveled and dressed....and/or the A string raised at the bridge saddle. Better to pay a pro than to make bad guesses. "Fixing" the wrong thing can have a domino effect and make things worse.
raise that string on the bridge a little or take it to a shop