- You fell for a scam and aren't actually using coinbase, but a phising site instead
- You forgot to actually create the transaction from your coinbase account (usually forgetting to click the button to actually create a transaction)
- Coinbase didn't broadcast the new transaction (yet) (this happens when you use centralised webwallets)
- You entered the wrong address when creating a transaction in your coinbase wallet (usually a copy/paste virus)
- The transaction you created in your coinbase wallet was rejected (for example due to dust outputs or insufficient fee)
- You're running a fake electrum version (did you check the signature?)
- Electrum isn't connected to a node
- You opened the wrong electrum wallet
Some of these things look trivial, but i'd still advice to check them all off...