So what am I doing wrong? My Ubuntu box, tablet and phone all seem to function like completely separate wallets. I am using the same seed - is there something I'm missing?
When it says synched, it means addresses, transactions, amount of bitcoins, etc.
Labels, contacts, and other things not related to your wallet's addresses will not be synched.
To test the synching functionality, send some bitcoin from one address in your wallet to another wallet on your tablet etc.
Once the transaction has been broadcasted, load up electrum on another device with the same seed... it should show that transaction from your tablet on your other device as well.