You need clean any residual solder and flux from buttons you harvested from your parts only board. Use solder wick to clean all the J leads on the button.

Warning: you can damage the casing and leads on the button if you remove it incorrectly or apply to much heat.

Tip: tip check the operation of the button to be sure it wasn't damaged in the removal process.

Tip: make sure you clean any excess flux from the button.