Hello, I have rummaged through previous similar issues and tried a few things.
I’ve tried restarting Bisq a few times, then tried resyncronising the spv, restarted Bisq twice as per instructions but my payment confirmation is still not going through.
I’ve tried messaging the seller but the message is not being received.
I’ve tried looking at how to contact a mediator but there is no option for that on my screen, the only option is to open the chat with the seller.
What else can I try? 
Hello
I understand you already checked this?
By the message in trader chat not being received, what do you mean? What does the notification at the bottom of the chat dialogue say?
Usually, the problem with the payment started button occurs when the peer is offline, so by the same reason, they will not directly receive the message, which might instead get to their inbox.
Please give additional details, maybe redacted screenshots.
Check the deposit confirmation transaction ID. If it is not found on a blockchain explorer it means the trade has failed and you should open mediation (Select the trade and clinic Ctrl + O).