What could be the reason for a lower yield when using M2 Macrophage Generation Medium compared to M1 Macrophage Generation Medium for a culture started with the same monocytes?

For the M2 Macrophage Generation Medium; it is extremely important that the shelf life of 2 weeks (after addition of the cytokines) is not exceeded; the yield will quickly decrease thereafter. It is best to use the M2 medium as fresh as possible to avoid discrepancies between M1 and M2 yield.