Down shifting issues sometimes happen on folders due to the long cable runs. They are therefore sticker than the shorter runs on ridgid bikes.
Solution are either to make the cables more slippery or increase the spring return.
I tend to fit telfon coated inner cables and telfon linned outers with a tad of oil.
To increase the spring you can add a pen spring to the cable terminals or use a hair bobble attached to the drailler cable secure bolt.
Try a tad of oil and shift gears for several minutes. You may find that's all it needs. I tend to have problems with my black mezzo if it's not been used for a while.