Does the pot drain?
How often do you water, and how much?
Reading up on them I see several articles saying Syngoniums want high moisture but have thin roots and depend on the soil for that (compared to a Spider plant which will damn near split your pot with massive roots that can sustain the plant for weeks+).
They also recommend regular misting though, so I'd take all of this with a grain of salt.
Misting can't hurt but for me has never solved a humidity issue.
