First of all, I had an existing installation which is about 25 years old consisting of a 10/2 wire all run which used to feed an electric motor for a saw. It was connected to a 30 amp double pole breaker in the box so we're on 220 volts.

With all saw equipment removed and only the 10/2 feed from the breaker box remaining, I wanted to feed 10' of electric baseboard. I started by cutting the 10/2 wire and installing a double pole thermostat. At the end of the 10/2 wire, I connected a 4' electric baseboard and then jumped this to a second 6' long unit.

My problem: the 4' unit seems to work OK and gets pretty hot as it should, however the 6' unit only gets warm, not hot like the 4' unit. I checked my connections and they seem to be tight. I replaced the 6' unit with another one and it performed just the same. Suggestions?

Are both heaters rated for ~240 volts?
Are you sure you have wired them in parallel? If you have wired them
As a series circuit then what you describe could happen...