PX4 code changes is not reflected when re-built

Hi All,
I am making changes to some files in src/modules/. However, the code changes are not reflected when I build PX4 as follows:

HEADLESS=1 make px4_sitl gazebo_solo

I am using Gazebo simulator and QGroundControl. So, basically I do not see the effects of the code changes in QGroundControl map view. I am assuming that probably I am not building the code properly. May be removing the previous gazebo builds could fix the issue. But I cannot figure out how I can remove the previous builds.

Any input here is highly appreciated!!