Leaf yellow dropping and brown spots on stem and leaf
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While this sounds like overwatering, it's hard to say for sure without photos. What does the soil feel like each time before you water? ZZ plants like their soil to almost completely dry out between waterings. If the soil is staying consistently damp, it will start to compact around the roots, cutting off their oxygen supply and suffocating them. That's what leads to root rot. If this is in fact overwatering, you'll need to pull the plant out of its pot and examine the roots. Rotten roots will be darker, mushy, and easily fall apart when handled. These will need to be removed before repotting in fresh soil. It's also a good idea to mix in plenty of perlite with whatever potting soil you use. It helps improve drainage and creates air pockets throughout the soil, allowing more oxygen to reach the roots. Do 3 parts potting soil and 1 part perlite.
