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On my Xperia keyboard settings, I have selected "Correct misspellings automatically or get word suggestions" and "Learn and back up my writing style". In "Use my writing style" I have selected "Learn from Messaging" and "Backup and sync" and in "My words" I have many words the phone has saved.
BUT when I type in any app, the keyboard doesn't recognise any of the learnt words and when I check in the settings option on the keyboard, and select "My words", there are no words there!
I didn't have the problem 2 weeks ago - how can I fix this?
I'm having the same problem. The keyboard does not want to learn new words (in any app/ sms/whatsapp/etc...), and delete all the saved ones.
It would be worth resetting the keyboard itself in this case: Main Menu > Settings > Apps > All > Xperia Keyboard > Clear Data.
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I have the same problem with the Xperia keyboard. It saves words for a period of time and suddenly it deletes them and some times won't learn new ones and the "My Words" stays empty until I clear the Keyboard data. I tried to clear data and even factory reset the phone but it saves words for a wile and then the list is cleared and i lose all my words. I had this problem from the beginning and is very frustrating to build up a dictionary in time and one day all the words disappears for no reason.
I am waiting for someone from Sony Xperia Support to forward this issue to the developers. Or an answer if this issue is happening in Android 5.0.2 because in 1 or 2 weeks Sony starts to roll the lollipop for Z1
It's been over a week and 2 days since the lollipop update and My Words are still there. I hope the issue was fixed. In the 4.4.4 version the words disappeared after 1 - 2 weeks so until another 1 or two weeks from now i can't be sure if the issue is still there.
Had a few issues with the keyboard but believe them to lie in the fact that I have other keyboards installed along side the standard one.

Check with you other keyboard app developer for support on this issue too
hope this helps
After a month i believed that the issue was fixed but this weak bang!! all my words are gone so the issue is not gone.
There is one possibility not to lose all words and start from the beginning, if you keep all sms messages that you sanded and make the keyboard relearn from them, but this is annoying.
Come on Sony fix the keyboard.

Mine started acting up 1 week ago, after updating software.
So frustrating.
I have to keep keying the alphabets individually (I use keypad type keyboard)
Sony... deal with this problem.
I will see the list in "My Words" after I click "Learn from Messaging".
I'll be able to enjoy interruption free messaging for awhile.
It'll disappear after that. Go back in to check "My Words", the list disappeared.
This needs fixing!!!!
I had this issue before I used the Entertainment Network or any services of this kind...
In the lollipop version the problem occurs less often then in 4.4.4 but is still there.
Until the issue is fixed....
the only workaround that is : Building a personal dictionary by keeping all the sent messages and making sure that you type them correctly when you write them and when a sufficient number of sms messages are sent you almost have your dictionary. Then use Backup and Restore app that came with your phone and backup only your sms messages and keep the backup on the phone.
If the keyboard bug manifests itself again just relearn the words from messaging if you still have the sms or restore the backup of the sms that you did initially.
For me the problem occurs once a month and i don't need my sms messages so i gather as much as i can so that the dictionary is complex enough and then I backup regularly. After I learn the words again i can delete all conversations from messaging app and only keep them in a backup copy...