What does 15-day Premium Processing clock stopped mean?

I have received my premium processing receipt number RQ/PP/CSC on 11th April. On 19th April the status showed as Intial Review with following comment.

“On April 18, 2013, the 15-day Premium Processing clock stopped on your I129, PETITION FOR A NONIMMIGRANT WORKER. A notice was sent on this date explaining the specific reason for stopping the premium processing of your case. This notice will contain detailed information on further action required by you. Your premium processing receipt notice contains contact information for direct inquiries on your case.”

It is in the same status since then. I have seen quite a few such cases when I browsed through using different receipt numbers. Any idea what is going on here?

If the status doesn’t change, then check w/ your attorney and ask them to follow-up w/ USCIS. This could be b/c of RFE or approval.