We are guessing without your complete code and a schematic of your keyboard.

There are several issues to be considered when designing a [switch matrix][1] for an embedded processor regardless if it is a musical or computer keyboard:

 1. [Contact bounce][2]: This is more important for a computer keyboard
    then a musical keyboard.  However if the switch are particularly
    bad, it may audible.  Software may be used to mitigate the problem
    but will also add delay between the key press and the beginning of
    the note's sound.
 2. [Pull Ups][3]: Any logic input should not be left unconnected or
    floating.  This can lead to unexpected results.  Use Pull Up
    resistors (or Pull Down depending on your design) to mitigate this
    problem.  The [Arduino Uno's][4] embedded processor ([the
    ATmega328P][5]) comes with built in Pull Up resistors.
    The Arduino IDE abstracts this feature by including a parameter it
    the [pinMode() function][6] call to invoke this feature.
 3. [Rollover][7]: In a switch matrix of m rows and n columns a single
    switch closure can be uniquely detected.  However, if two switch
    closures occur in the same row or column, they can not be uniquely
    detected.  This is because current can flow through both closed
    switches activating both rows or both columns nullifying the ability
    for the embedded processor to scan individual rows or columns.  To
    mitigate this problem a diode is added to all switches in the switch
    matrix such that there is no current path between rows or columns.

From the [Rollover wikipedia page][8]:
> Most music keyboards use isolation diodes in their keyboard matrix to
> implement full n-key rollover, making them immune to both key ghosting
> and key jamming


This image if from [this stack exchange question][9]:
[![This image is from [this stack exchange question][8].][10]][10]


  [1]: https://en.wikipedia.org/wiki/Keyboard_matrix_circuit
  [2]: https://en.wikipedia.org/wiki/Switch#Contact_bounce
  [3]: https://en.wikipedia.org/wiki/Pull-up_resistor
  [4]: https://store.arduino.cc/usa/arduino-uno-rev3
  [5]: http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-Automotive-Microcontrollers-ATmega328P_Datasheet.pdf
  [6]: https://www.arduino.cc/reference/en/language/functions/digital-io/pinmode/
  [7]: https://en.wikipedia.org/wiki/Rollover_(key)
  [8]: https://en.wikipedia.org/wiki/Rollover_(key)#n-key_rollover
  [9]: https://arduino.stackexchange.com/questions/31885/need-help-in-circuit-diagram-of-88-reed-switch-matrix
  [10]: https://i.sstatic.net/76A84.jpg