Brilliant, thankyou - restarting the app did the job. Top advice.  

Hi - newbie question....
I'm trying to transfer BTC from a web-based wallet (localbitcoins) to my Electrum 2.7.18 watching-only wallet.  But when entering the Send To address, I entered an address which my Electrum wallet had used to receive an earlier payment, instead of generating a new request and address.
The transaction was initiated about 4 horus ago, and currently shows on the blockchain website as having 24 confirmations - but it hasn't shown up anywhere in my Electrum client as an unconfirmed transaction.  The Electrum client is online.
Can I somehow force my Electrum client to check for incoming payments to an old address, or should it do this automatically? 
I know that it's best not to re-use bitcoin addresses, but I thought that all the addresses generated by my Electrum client/wallet would always work with that wallet.
Grateful for any advice, thanks.