When I am given off to approach and on final with the APP on and the NAV1 on the right freq. Sometimes I can't get established on the localizer. Most times I have to go missed. Tonight I flew from KAUS to KMSY. They gave me rwy 19. After I was heading toward the rwy and the ATC told me to maintain 180 it stayed there. It did not lower me and this was 4 miles out. I even tried a short final on rwy 28 and had the same results. I had to just shut down FS after 3 misses. Any help on this issue is needed.