Welcome to Android Central! Does the SwiftKey keyboard just not come up? Or does the tracing not work? Can you give us more details?
Try going to Settings>Apps, select SwiftKey, and Clear Cache/Clear Data, then try again. Or you could try uninstalling SwiftKey and then reinstall it again.
If you get a different keyboard, check that SwiftKey is still set as the default. Depending on your phone it is usually under something like Settings>Language & input>Default.