When I activate a scene from an Insteon switch, it works perfectly. When I ask Alexa to execute only some of the actions in the scene ate executed

If you activate the scene from the admin console, does it behave the same as with Alexa?

Likely the answer is yes. Which means your scene links are corrupt. Try restoring all the devices in the scene. If that doesn't work, trying removing the device that isn't responding from the scene and then adding it back in. If that doesn't work, start deleting devices, factory resetting them, adding them back to ISY, then creating the scenes again.