sometimes Conferencing participants are dropped from Audio conference , there are known issues that is caused by McAfee (HIPS software scans) and can be resolved by installing the latest patch.
If it is your case, please download and apply the latest HIPS version 7.0.0
patch by contacting the vendor company.
http://download.nai.com/products/naibeta-download/mhip/HIP_700_ReleaseNotes.html
Mino - The UC Guy 