The Serial.parseInt() and Serial.parseFloat() functions will return the next valid number in the incoming serial, or return zero if a valid number was not found.

If your loop() function is calling the parsing function when Serial.available() == 0 returns true, and you are sending a number value through the Serial Monitor with a line ending configured:

1. Read the first valid number value in the buffer. Line ending characters will be left in the buffer, and Serial.available() == 0 will return false.
2. If the function is called again before a new number value has been added to the buffer, it will return zero.

You can solve the problem by:

• Selecting No line ending in the Serial Monitor.
• Using the Serial.read() and define your own logic for reading the buffer.