By far the most common issue with the Muscle SpikerShield is that the pins are not making proper contact with the Arduino. This can cause a wide variety of problems and strange behavior.
The first thing we should try is separating the SpikerShield and Arduino, and bending the pins slightly, as seen in this video.
After that, make sure that the SpikerShield is pressed firmly into the Arduino board, and pull very slightly up.
Updating the Firmware can also be a good idea if you have an older model, and is also a good variable to eliminate when troubleshooting. You can find all of the firmware versions on The Muscle SpikerShield Github Repository
Check out this topic to see how to upload code onto an Arduino