you require a neutral wire for your lights. terminate the wire going from dimmer #1 to the lights(bottom dimmer in your diagram). connect a neutral wire to the lights power from the location that used to go back to dimmer #1. the black terminal of dimmer #1 should have only 1 connection, that being constant power. the lights get their power from the black terminal of dimmer #2 (top dimmer in your diagram) and require a return path of current to neutral. use 14/2 wire from your constant source to the dimmer #1 location, then connect the neutral from that 14/2 to the light power. you don't show what cables you are using in your dimmer setup.

The most common wiring problem is that with these dimmers is switching the order of the Master and Slave switches. The order is breaker-box to slave to master dimmer to light. I'm concerned that you checked for a "hot" wire after you have installed the switch. You need to do that before anything is hooked up. Once the switch is installed, any number of wires may be "hot". It's never a good idea to assume anything about wires. The "neutral" wire (that you called "common"?) will normally be white. From the breaker side of things, you'll have a hot, neutral and ground. They're almost always black, white, and bare (or green) copper. These wires are connected to the black, white and green wires of the slave switch. Then, there's a cable that goes to the other switch location. It typically has a black, red, white, and bare (green) wire. In the slave junction box, the black, white, and bare wires get connected to the power black, white and bare wires (the same as the slave switch). The red wire won't be used, so can be capped for future use. In the Master switch box, connect the black, white, and bare wires coming from the slave box to the black, white and green wires on the Master switch. Again, the red wire from the slave box won't be used, so can be capped off. Then, locate the cable going to the light fixture. The white and bare wires from this cable will connect to the white and green wires on the Master switch. The only difference here is that the red wire from the Master switch goes to the black wire feeding the light. If this doesn't help, pictures of the two boxes might help to figure out what isn't right.

Are the light you are trying to control now controlled by a single pole switch? Are you replacing that single pole with a 3 way and adding a 3way at another location? If yes to both, then connect the incoming hot to the common (black) screw of the new 3way at the existing box, connect the black of the 14/3 to the black going to the light, and connect the red and white of the 14/3 to the other two screws of the 3way switch. Leave the whites already there alone. At the new location, connect the black to the common (black) screw and the red and white to the other two screws.

From the 2nd switch you only need two conductors, a hot and a neutral, going to the light. The hot will come from the "common" terminal on the switch. No reason to replace the cable, just leave the 3rd wire disconnected. Whomever wired it to begin with probably either had extra 14/3 around, or wanted flexibility, for example to power a fan and a light separately.