USCIS Limitations on I-140 Premium Processing

From the USCIS Website:

Are there any additional conditions of availability being placed on the Premium Processing Service at this time?

Yes. This will accord USCIS the flexibility to adapt to
contingencies affecting its ability to provide Premium Processing
Service. Premium Processing Service is available for the Form I-140
classifications indicated on the chart above provided that the case
does not involve:

1. A second filing of a Form I-140 petition while an initial Form I-140 remains pending;

2. Labor Certification substitution requests; and

3. Duplicate Labor Certification requests (i.e., cases filed without
an original labor certification from the Department of labor).

USCIS is prescribing these additional conditions of availability on
Premium Processing for Form I-140 because of their special processing
requirements, including locating and transferring other files or
documents internally and requesting initial evidence from an outside
agency, that make it difficult for USCIS to guarantee that it will
process the case within a 15 calendar day period.