Confirm the source of the issue. Run a coalescence test on both lots of paste.
A simple coalescence test can quickly determine the condition of solder paste after prolonged use. Simply print a small disk [around 4-5mm diameter and 0.2mm thick - a business card with a hole from a standard paper hole punch makes a good stencil] of paste onto a non-wettable substrate [eg, glass microscope slide, solder mask on board, ceramic tile], and re-flow as normal. * A single solder ball in a clear pool of residue indicates good coalescing ability. * Numerous solder balls remaining in the flux residue pool indicates poor coalescing ability and the paste may have become damaged.
Note this is not an absolute test of the condition of the paste. It is only a first-line check and should only be used to confirm paste failures. [based on Multicore Solder Paste Handling Guidelines, September 2004]