Solder paste not wetting at all
My guess would be that the copper board is not being given enough time to heat up. Due to its thermal mass the copper heats up much more slowly than the solder, and the solder melts before the board reaches the correct temperature. If you choose a smaller piece of copper, or an etched PCB with less copper on it, or leave the copper board in the reflow oven longer, the solder will eventually flow as expected. It's probably just that the large thermal mass can't heat up enough before the solder melts.