I am having the same problem, this problem started like 4 to 5 days ago.
i have just contacted the Samsung Support, i supplied him with following query,
i am using GT-i9505, and having problem with Samsung Keyboard, Specially Personalized Text, don't remember words,
he did his best resolving the issue, by following steps, (performed through Samsung Tutor) must have the predictive text option on, through Language and input settings, also the Personalized text must be active.
1. he go to settings, More>Application Manager>(Tab) All> selected Messages> and cleared the data by clicking on Clear Data,
2. Clicked on "More" button, (left touch key beside Home Button") and selected Clear App Preferences.
3. asked me to restart the phone, by shutting down and Removing the battery.
4. restarted, tested, worked.
but as soon as he left, again doomed
i tried the word "Asalam o Alaikum" he told me you will have to retype and save the words again, so, at first it was working, i would only write "asa" a suggestion will appear as Asalam, i press o then Alaikum would automatically appear.
but its not working again.
i suppose i need to reset the phone, but i think it can be worked out as,
Settings>application Manager>all (Tab) select Samsung Keyboard> clear Data, (mine was 49 mb) think its working for me as of now.