This is a common issue which indicates either an issue with the underlying mesh or the platform scheduler. If the mesh loses a packet then the command never reaches the thermostat (this can happen for a lot of reason which are explained in other FAQ topics) or if the platform timer skipped an execution due to heavy load it could also result in the same situation.
You may want to address the underlying mesh issues if possible, check that your thermostat was paired with the C-Wire connected as explained on this topic first post.
Alternatively you can use a hardier app like this one which does come verification to handle these types of underlying issues: