I'm having this problem continuously on an iPhone 4S which is running iOS 8.3 at the moment.
With no user command, the predictive text appears to switch itself off and no amount of swiping up down or in any other direction will restore it.
When I go to the globe key, predictive has a green (for ON) switch. I flick it off, return to the keyboard, go back to the globe, select predictive and switch it on. Most of the time that cures the problem, but sometimes, several switch-processes are necessary to get it back running.
This happens across the board in Mail and Messaging and knocks-on to Apps which I have installed from the AppStore.
It's a nuisance and I'm sure Apple never intended it to work to its own agenda like this, so help, please!