alcCaptureSamples() loss data

`alcGetIntegerv(app->capture_device, ALC_CAPTURE_SAMPLES, 1, &ncaptured) And alcCaptureSamples(app->capture_device, data, TEST_BUFFERSIZE)`

In Emscripten V2.0.16 , I get the capture data by alcCaptureSamples. I believe the the loop buffer pointer is incorrect in the generated .js file, so I changed the loop buffer pointer:
' function _alcCaptureSamples(deviceId, pFrames, requestedFrameCount)
{
.........
c.capturedFrameCount -= requestedFrameCount;
} '

But , Occasionally the PCM data still will be lost, and the sound will appear cicici .

In the Cool Edit Pro tool, you can see that the PCM spectrum will appear as a red vertical line ，that mean the data loss.

Do you know why?