Only problem I have and you will find this out also, is most keyboards on the tablets will not stay as your chosen keyboard. Every time I turn off my GN 10.1 or GN 8, it reverts back to the Samsung Keyboard by default. I just bought the Swype keyboard also, since it was on sale, and same problem. I love the Swiftkey flow on my phone, which works without any problems, but for some reason with the tablet versions of the keyboards, they all revert back to Samsung Keyboard. Supposedly it's a problem on Google's end with the Play Store, but why does it work fine on my Phone?
Anyways, I'm sure you've done this, but make sure the Flow option is checked in settings under swiftkey flow. Not sure about the predictions part, I don't see that in the settings. Did you try to uninstall and reinstall the app again. That may help. I had one app that froze my Note 8, then I uninstalled and reinstalled it and all is fine now.