I'm using SGminer to mine, and whenever I get an accepted, the wallet flashes a payment of 400 - but it disappears - nothing turns up. What am I doing wrong, please?
Nothing, they are orphaned blocks and are normal when difficulty is low. It just means that somebody found a new block a little earlier than you. Matter of luck, computing power and network latency combined.
Also during a launch of new coin make sure that your intensity is low (17 works fine here) and add --queue 0 --scan-time 0 arguments to sgminer command line. It reduces overall latency of your miner and therefore also number of orphans.