This is largely related to experimental (handling) error. It is recommended to place the pipette vertically with the tip against the bottom of the sample well in the cartridge when adding the mastermix to avoid creating air bubbles in the sample well. If air bubbles appear in the sample well, large bubbles will be created when generating droplets, resulting in a reduced number of droplets.
Also, please remember to always add oil first and then add the mastermix to ensure successful droplet generation. Nevertheless, it is always important to examine the goods upon receipt as well as monitor your storage temperature to rule out compromised quality due to logistics and storage.