In my years of experience with BP checks, not all of them were audible enough to convince me of an accurate reading all the time.
When you can't hear the brachial pulse after a few attempts, do the palpation technique (radial or brachial).
1. FEEL for the pulse (radial or brachial). As you inflate the cuff that pulse becomes faint.
2. As you deflate, feel for the first strong pulse and mark the reading.
3. Now put your steth back in the brachial site. Inflate the cuff about 40 mm hg above your mark.
4. Deflate slowly and listen for a brachial pulse. About 5-10 mm hg above (sometimes below) your mark you should be able to hear a pulse. It may be faint still but it's audible enough for a systole and diastole.
Frequent BP practice reading will improve hearing that elusive pulse.
More info on same issue on this thread.... (click on it)