You can't swipe with SwiftKey from the lockscreen. Nothing wrong on your end. The app doesn't allow this since Lollipop came out because of some changes Google made in the lockscreen that made it impossible for them to implement it. They won't fix it because they say they need to 'rebuild the app from the ground up'. Every time a user asks about it they say they have no intentions to addressing the issue.
Ever since Gboard came out I couldn't be happier. It has everything I loved about SwiftKey: multilanguage, number row, dictionary sync (except the arrow keys) plus I can swipe in lockscreen (a MUST in Android 7 and its reply from the notification feature). It's also far more accurate when swiping in both Spanish and English.
I was a big fan of the arrow keys for cursor control too, but Gboard's swipe in spacebar feature works great for me. I was inserting spaces at first as well, but you get used to it over time and learn to use it properly. Also, the gesture in the back key for deleting is big plus.
After 4 years using SwiftKey, I can say that I won't be going back.