There is a specific way to practice this. I struggled with this too and found this particular technique helpful. It essentially entails training the ear to associate known intervals with chords.
The standard way to practice intervals is to listen to a first note, then litsen to a second note, and then identify the interval between them. For example, you might hear
F4, and then say to yourself "that's a perfect fourth." The two notes can be ascending or descending--it's valuable to diversify one's practice and incorporate both into one's ear training.
This interval work can be done alongside ear training for identify chords. In particular, play the two notes (one after the other), and after identifying the interval, play the two notes together as a chord. If you are struggling to hear the bottom note, you might try playing the two notes in descending order, like this:
C4, then an
F4-C4 chord. As you hear the chord, sing the lower note from the interval (the
C4), and match that lower note you're singing to the tones you hear. This will train you to distinguish the lower note from the higher notes, and it will train you to recognize specific intervals (perfect fourths, major thirds, etc.) within a single chord.